Lamellibranch Mollusks of the Baidaevsky Deposit (Kuzbass)

L. L. Khalfin

Over the course of a number of recent years, geological organizations—first and foremost the trust “Kuzbassuglerazvedka”—have devoted attention to the study of one of the most widespread groups of the fossil fauna of Kuzbass, namely the lamellibranch mollusks. In connection with this, all geological parties working in Kuzbass organized systematic collections of this fauna; the extensive collections assembled in this way were subjected to detailed study, which made it possible considerably to expand and refine our information about this group of fossils [5]. The most substantial results were obtained with respect to the fauna of the Balakhonka and Kuznetsk suites, and partly of the Ilyinka suite, whereas the fauna of the Erunakovka suite remained far less studied. The Abashevo district, specifically the Baidaevsky deposit, is one of those districts of Kuzbass in which the Erunakovka suite is most richly characterized paleontologically. A natural continuation of the studies carried out earlier should have been the study of the pelecypods of the Erunakovka suite of this district, which is the subject of the present work. The accomplishment of the latter proved possible only because the Baidaevsky party of the trust “Kuzbassuglerazvedka” had collected a large collection of fauna in the indicated deposit; especially interesting and carefully conducted collections were made under the direction of E. M. Senderzon and G. M. Kostomanov, to whom the author expresses his gratitude for transferring these collections to him for study.

The literature data on the pelecypods of the Erunakovka suite are very scanty. Leaving aside several short articles by L. A. Ragozin, which require revision, and the work of L. L. Khalfin [4], in which pelecypods only from the very lowermost beds of the Erunakovka suite are described, we shall have at our disposal, for the fauna of the Erunakovka suite, only those data that are contained in two works by D. M. Fedotov [2; 3], who studied extensive collections assembled chiefly by M. F. Neuburg and V. I. Yavorsky. According to these data, nine species of pelecypods have been established in the Erunakovka suite (excluding forms identified approximately or reidentified by the same author); among them, according to D. M. Fedotov [3, 242–243], the following species are found only in the Erunakovka suite:

Anthraconaiа gapeevi (Fed.)
Abiella tomiensis (Rag.)
Abiella concinna (Jones)
Palaeanodonta tersiersis Fed.
Oligodon (?) plotnikovskiensis Fed.

However, Abiella concinna, according to the same author (Fedotov [2, 46]), also occurs in the Ilyinka suite, while “Abiella” tomiensis and Micodonta plotnikovskiensis (= Oligodon? plotnikovskiensis Fedotov) were found by me

in the composition of the fauna of the Il’inskaya suite from the last section along the Tom’ River near the village of Kazankova. Thus, only two species (Palaeanodonta tersiensis and Anthraconalia gapeevi) prove to be characteristic only of the Erunakovskaya suite. But the study of pelecypods of the Abashevo district has shown that the matter is by no means so unfortunate: in the Erunakovskaya suite of the Baidaevsky deposit a number of forms have been found that are significant guide fossils both for the Erunakovskaya suite as a whole and for some of its parts.

Before continuing the review of the pelecypod fauna of the Erunakovskaya suite, I shall characterize in general terms the material whose study formed the basis of the present work. It consists, first, of a collection assembled under the direction of G. M. Kostomanov; this collection comprises 144 core samples derived from 44 boreholes and covering the section of the Erunakovskaya suite from seam 14 to seam 38 inclusive. An important supplement is the collection supplied by E. M. Senderzon; it consists of 43 core samples characterizing the stratigraphic interval between seams 12 and 25. Thus, the fauna studied by me belongs to the three upper horizons of the Erunakovskaya suite of the Baidaevsky deposit according to E. M. Senderzon’s scheme—namely, to the Abashevo, Baidaevsky, and Esaulsky horizons—and only the very lowest horizon (the Fes’kovsky) is not represented in the collections available to me. The total number of pelecypod specimens amounts to thousands. The preservation of the fauna varies, in many cases being good, and some specimens, in their preservation, are unique for the fauna of the Erunakovskaya suite. However, with respect to its state of preservation this fauna suffers from the general defect characteristic of almost the entire fauna of the coal-bearing sequence of Kuzbass [5]: it is represented by external impressions and sculptured casts, that is, by the very nature of its preservation it does not permit the study of details of the internal structure of the shells, in particular of their hinge.

The fauna of the Erunakovskaya suite belongs to the freshwater Kolchugino complex of faunas, which, within the Kuznetsk Basin, replaced the Balakhonka complex of brackish-water faunas [5]; this replacement occurs at the boundary between the middle and upper (Chebalinsky and Tersinsky) horizons of the Kuznetsk suite, and the Tersian fauna is the earliest in time to belong to the Kolchugino complex. Higher up, in the Il’inskaya and Erunakovskaya suites, the Kolchugino complex reaches full development, but until now it has been possible to distinguish within it only two faunas differing in their general character: a normal Kolchugino fauna and a depressed Kolchugino fauna. The latter is characterized by an impoverished generic and species composition and, in its typical expression, by dwarf shell sizes; it is distributed chiefly in the very lowest beds of the Erunakovskaya suite, whereas the normal Kolchugino fauna is abundantly represented both below, in the Il’inskaya suite, and above, throughout the entire extent of the Erunakovskaya suite. Until the present time, however, there have not been sufficient data to divide this fauna into two, corresponding to the two suites indicated above. The reason for this is, on the one hand, the abundance of common forms in both suites, and, on the other, the insufficient study of the fauna of the Erunakovskaya suite. The study of the pelecypods of the Baidaevsky deposit makes it possible to take a definite step toward establishing differences in the composition of the fauna of the Il’inskaya and Erunakovskaya suites.

The Kolchugino complex as a whole is characterized by a considerably more uniform and poorer generic composition of pelecypods than the Balakhonka complex; in the latter the presence of 16 different genera has been established, whereas in the former only 7 genera have been recorded in all. Of these, the most typical and at the same time the most widespread are the genera Microdontella Leb. and Microdonta Khalif., especially the former. Extremely widely dis—

the genus Anthraconauta Pruv., which, as we shall see below, constitutes the most important group for the biostratigraphic subdivision of the deposits of the Ilyinka and Erunakovskaya suites. In places, representatives of the genus Palaeanodonta Amal. occur in abundance; more rarely, representatives of the genus Abiella Rag. emend. are found, which is absent in the Balakhonka complex, and only at one locality (in the deposits of the Ilyinka suite of the Prokopyevsk district) shells of the genus Neannigenia Khalf. have been found, distinguished by unusually large dimensions; the genus Anthraconaja Tr. et W. also occurs rarely. All these genera have been characterized by me in a recently completed work [5].

A characteristic feature of the Kolchugino complex, in comparison with the Balakhonka complex, is the extremely broad vertical distribution of a number of forms; among them are also the most numerous in number of specimens and the most widely distributed within the development of the deposits of the Ilyinka and Erunakovskaya suites: the species Microdontella subovata (Jones) and Anthraconauta pseudophillipsii Fed., as well as a number of others. This gives the pelecypod fauna of both suites such a great similarity, which is further intensified by the fact that the species characteristic only of one or the other of these suites differ by features that are far from obvious and not conspicuous.

The general list of forms established by us in the Erunakovskaya suite of the Baidaevsky deposit is as follows:

1. Anthraconauta pseudophillipsii Fed.
2. Anthraconauta pseudophillipsii var. acuta Khalf.
3. Anthraconauta pseudophillipsii var. angusta n. var.
4. Anthraconauta pseudophillipsii var. subaequalis Khalf.
5. Anthraconauta cuneata n. sp.
6. Anthraconauta tchernychevi Khalf.
7. Anthraconauta n. sp., cf. A. tchernychevi Khalf.
8. Anthraconauta ampla n. sp.
9. Anthraconauta propinqua n. sp.
10. Anthraconauta kostomanovi n. sp.
11. Anthraconauta tenuis n. sp.
12. Anthraconauta subparallela forma postera nova
13. Anthraconauta subparallela forma nana nova
14. Anthraconauta inflata n. sp.
15. Anthraconauta simplex Khalf.
16. Anthraconauta lata Khalf.
17. Microdontella subovata (Jones)
18. Microdontella subovata var. elongata Khalf.
19. Microdontella subovata var. megala Khalf.
20. Microdontella cyclos Khalf.
21. Microdontella elliptica Khalf.
22. Microdontella (?) protumida n. sp.
23. Microdonta baydajevskiensis n. sp.
24. Microdonta longa n. sp.

Examination of this list shows that, in the composition of the studied fauna, anthraconautids predominate (2/3 of the entire composition); however, if one turns to the quantitative development of the various forms, then in first place (as is generally characteristic of the Kolchugino fauna) will be the species Microdontella subovata, representatives of which are present in great numbers in very many faunistic beds. The list given above, consisting of 24 names, shows that the fauna of the Baidaevsky deposit considerably expands our knowledge of the fauna of the Erunakovskaya suite in the ce—

...for which, according to D. M. Fedotov (see above), only nine species were known in all. But the following circumstance assumes still greater significance: our list lacks most of the forms cited by D. M. Fedotov, and representatives of the genera Abiella, Palaeanodonta, and Anthraconia are entirely absent. This circumstance requires consideration and some, if only provisional, interpretation.

Let us note first of all that the fauna of the Baidaevsky deposit shows no signs of a depressed state; this is indicated both by its mass development and by the large size of the shells—often even larger than usual: the pelecypods of the Baidaevsky deposit give a definite picture of a flourishing fauna. It is hardly possible to explain the absence of the genera mentioned above solely by accidents of collecting; against such an assumption speaks, above all, the abundance of the collected fauna, although with respect to the genus Abiella such an assumption cannot be excluded altogether: this genus, so far as I can judge from my own experience, usually occurs as isolated specimens, which may not have entered the collection being characterized, although in the deposits of the Ilyinskaya suite of the same Abashevo district they have been found (see Table III, fig. 13).

The absence of palaeanodonts may perhaps be explained by causes of a facies character: apparently, these long and narrow shells lived predominantly in running waters (in rivers) and in bodies of water connected with them, whereas (this will be discussed in more detail below) the rocks containing the fauna of the Yerunakovskaya suite of the Baidaevsky deposit were formed predominantly in standing or weakly flowing basins of a lake type. On the other hand, one must not lose sight of the fact that the collection I studied does not cover the entire section of the Yerunakovskaya suite: first, as already noted, I lack the fauna of the lower (Fes’kovsky) horizon, and second, in other districts of Kuzbass deposits are known which apparently occupy a position higher than the Esaulsky horizon of the Baidaevsky deposit.

Thus, the absence from our collection of one or another of the forms among those indicated by D. M. Fedotov is probably explained by various reasons. But in any case this compels me to note that, despite the extensive and new material supplied by the collection I studied, we still cannot assert that the composition of the pelecypod fauna of the Yerunakovskaya suite has at present been clarified in a sufficiently exhaustive manner.

Turning to the stratigraphic analysis of our fauna, let us note first of all that it includes a number of forms passing into the Yerunakovskaya suite from the Ilyinskaya and, consequently, lacking stratigraphic significance; these include:

1. Anthraconauta pseudophillipsii with two varieties
2. ” tchernychevi
3. ” n. sp. cf. Anthr. tchernychevi
4. ” simplex
5. ” lata
6. Microdontella subovata with two varieties
7. ” cyclos
8. ” elliptica.

All the remaining forms, sixteen in number, being new, have not hitherto been found in the Ilyinskaya suite and, consequently, may be regarded as guide fossils for the Yerunakovskaya suite. This conclusion is supported by the circumstance that, simultaneously with the study of the fauna of the Yerunakovskaya suite of the Baidaevsky deposit, I...

very extensive collections of pelecypods from the Ilyinskaya suite of the same region have been studied, not to mention the previously studied collection, gathered bed by bed by Yu. A. Zhemchuzhnikov and his colleagues from the section of the Ilyinskaya suite near the village of Kazankova on the Tom. Nevertheless, not all these 16 forms are equally valuable and reliable as guide fossils. Thus, some of them—for example, Anthr. cuneata, Anthr. inflata, and Microdontia longa—occur very rarely, in single specimens, and for this reason alone cannot be of considerable stratigraphic interest. Others, such as, for example, the variety angusta of the species Anthr. pseudophillipsii and two new varieties (postera and nana) of the species Anthr. subparallela, can be distinguished with difficulty, and only when sufficiently well preserved, i.e., by no means always, from forms widespread in the Ilyinskaya suite as well; consequently, in practical work, when the fauna encountered is scanty and poorly preserved, these forms cannot provide reliable grounds for stratigraphic conclusions. But alongside them our collection also contains forms that are more trustworthy in the indicated respect; these will be the following species:

1. Anthraconauta ampla
2. — “ — propinqua
3. — “ — kostomanovi¹
4. Microdontella (?) protumida
5. Microdonta baydajevskiensis

To these should be added the species indicated by D. M. Fedotov:

1. Anthraconia gapeevi
2. Palaeanodonta plotnikowskiensis

This, then, will be the principal guide complex of pelecypods of the Erunakovo suite; on the other hand, the study of other collections and the comparison of the results obtained with the results of processing the fauna of the Baidaevsky deposit make it possible to establish a guide complex of pelecypods for the Ilyinskaya suite as well; these will be, above all, the species:

1. Anthraconia (?) fedotovi (Khalf.)
2. Anthraconauta sphenoidalis Khalf.
3. — “ — obliqua Khalf.
4. — “ — trigonalis Khalf.
5. Palaeanodonta kuznetskiensis Fed.
6. — “ — pseudolongissima Khalf.
7. — “ — glossitiformis Khalf.

Of course, subsequent studies will have to expand and refine the faunistic characterization of both suites, but even now, on the basis of the data set forth above, it seems advisable to divide the normal Kolchugino fauna into two faunas—the Ilyinskaya and the Erunakovo faunas. In addition to the presence in each of these faunas of a definite association of guide species, the following differences of a more general character may be noted between them: palaeanodonts in the Ilyinskaya fauna are more widespread and more diverse, while anthraconauts in the Erunakovo fauna are, on the whole, larger in size; this applies especially to those anthraconauts that are typical of this fauna.

The stratigraphic distribution of pelecypods in the Erunakovskaya suite is shown graphically in Fig. 1, and for the most important group—the anthracosiids—the corresponding data are additionally given in Table 1. The following explanations should be made concerning Fig. 1:

1. The species Anthr. ampla and Microdonta baydajevskiensis pass through all three horizons and thus are guiding forms for the entire section studied; moreover, Micr. baydajevskiensis is most typical and most abundantly represented in the roof of seam 25.

2. The very lowest parts of the Abashevsky horizon (seam 12) are characterized by a very typical Anthr. subtenuis, possessing a very narrow and long shell.

3. The upper beds of the same horizon (seams 22–25) are characterized by Anthr. pseudophillipsii var. angusta, Anthr. cuneata, and Microdonta subparallela forma nana; the latter occurs in large numbers in one of the beds above seam 25.

4. A very important form—Anthr. propinqua—appears in the uppermost parts of the Abashevsky horizon and passes into the lower parts of the Baydaevsky horizon (seams 25–27a).

5. The lower part of the Baydaevsky horizon (seams 26–28) is characterized by Microdontella (?) protumida, whose small shells have been encountered only here, but in a number of beds in large numbers.

6. For the very top of the Baydaevsky horizon (between seams 32 and 33), one may indicate species that are, though rare, very characteristic—Anthr. inflata and Anthr. kostomanovi.

7. The boundary beds between the Baydaevsky and Esaulsky horizons (above and below seam 33) are characterized by the presence of the species Microdonta longa.

8. In the Baydaevsky and Esaulsky horizons as a whole, Anthr. subparallela forma postera is distributed.

All these data show quite convincingly that stratigraphic subdivision of the Erunakovskaya suite on the basis of the pelecypod fauna is by no means so hopeless a matter as might have been thought from the published materials, especially since in the Kazankovo section of the Ilyinskaya suite the upper and lower horizons are also rather clearly distinguished by this fauna [5]. Of course, the data presented above should not be overestimated: although they are based on a fairly large collection, it was collected only incidentally during exploratory work. One must also bear in mind the strong variability of the deposits of the Erunakovskaya suite laterally, with which, naturally, changes in the composition of the fauna are also connected. Consequently, the present work should be regarded only as a first step toward the zoostratigraphic subdivision of the Erunakovskaya suite; investigations in this direction must be continued in order to obtain a clearer and more definite picture of the vertical and horizontal (depending on facies changes) distribution of the fauna in the deposits of this suite.

Turning to the facies characterization of the deposits containing the fauna studied, let us note first of all that the data obtained once again confirm the complete absence in the deposits of the Erunakovskaya suite of any elements of marine fauna; the Erunakovskaya fauna (like the Ilyinskaya) has every basis for being regarded as purely freshwater. On the basis of the abundance and diversity of this fauna and the large size of most pelecypods, it must be acknowledged that this fauna lived under favorable conditions; in this respect it represents the complete opposite of the impoverished Kolchuginskaya fauna distributed in some areas of the Kuzbass [5].

In most cases the shells or their isolated valves are complete, not fragmented, which indicates their burial in

DISTRIBUTION OF ANTHRACOMYIDS
in typical sections of the coal-bearing series of the Baidaevsky deposit

Vertical stratigraphic labels visible along the column:

• [[unclear: stratigraphic unit]]
• [[unclear: stratigraphic unit]]
• Baidaevsky
• Tom-Usa
• [[unclear: stratigraphic unit]]

Visible taxonomic labels in the distribution diagram:

• A. rectangulata var.
• A. makarovskensis [[unclear]]
• A. aedilis var. angusta
• A. karpinskii var. rectangularis
• A. lineata
• A. [[unclear]]
• A. [[unclear]]
• A. cf. A. [[unclear]]
• A. [[unclear]] sp.
• A. [[unclear]]
• A. [[unclear]]
• A. [[unclear]]
• A. [[unclear]]
• A. [[unclear]]
• A. [[unclear]]
• A. [[unclear]]
• A. [[unclear]] var. [[unclear]]
• A. [[unclear]]
• A. [[unclear]]
• A. [[unclear]]
• A. [[unclear]]
• A. [[unclear]]

Scale: [[unclear: scale label and units]]

Fig. 1. Explanations in the text; the figures to the right of the column are the serial numbers of the coal seams.

Distribution of anthraconauts in the Erunakovo Formation of the Baidaevsky deposit	Distribution of anthraconauts in the Erunakovo Formation of the Baidaevsky deposit	Distribution of anthraconauts in the Erunakovo Formation of the Baidaevsky deposit
Name of forms	Stratigraphic position	Note
Anthraconauta pseudophillipsii Leb. f. typ.	Between beds 12 and 13; roof of bed 14; 1.5 m above bed 20; roof of bed 22; roof of bed 25; 12.55 m above bed 26b; 12 m below bed 26v; 19.45 m below bed 33; 19.27 m below bed 33; 28 m above bed 33; 37.55 m above bed 33; 45 m below bed 34.	Mass occurrence in beds: 1.5 above bed 20; 28 m above bed 33; 37.55 m above bed 33.
Anthraconauta pseudophillipsii var. acuta Khalf.	18 m above bed 25; 12 m below bed 26v; 6–8 m above bed 26v.
Anthraconauta pseudophillipsii Fed. n. var. angusta	8.4 m below bed 23; 28.75 m above bed 24; roof of bed 25.
Anthraconauta pseudophillipsii Fed. var. subaequalis Khalf.	6.7 m below bed 16; 15 m below bed 33; 55 m above bed 33; 55.35 m above bed 33; 56 m above bed 33.
Anthraconauta pseudophillipsii Fed. s. l.	Roof of bed 24; 6 m above bed 25; 81.5 m above bed 32; 15 m below bed 33; 28 m above bed 33; 30.9 m above bed 33; 83 m above bed 33; 89 m above bed 33; 92.55 m above bed 33.	Mass occurrence in beds: 81.5 m above bed 32; 15 m below bed 33; 28 m above bed 33; 30.9 m above bed 33; 92.55 m above bed 33.
Anthraconauta cuneata n. sp.	25 m above bed 22.
Anthraconauta tchernychevi Khalf.	30 m above bed 25; 18.65 m above bed 26a; 15 m below bed 33.
Anthraconauta n. sp. cf. A. tchernychevi Khalf.	10.1 m above bed 26b; 11.28 m above bed 26b; 20.85 m above bed 26b; 19.45 m below bed 33.
Anthraconauta ampla n. sp.	Roof of bed 12; roof of bed 14; 18 m above bed 25; 30 m above bed 25 (cf.); 29.45 m above bed 26b; 19.45 m below bed 33; 37.55 m above bed 33; 66 m below bed 34; 45 m below bed 34 (cf.).
Anthraconauta propinqua n. sp.	19 m below bed 26a; 15 m below bed 26a; 16.1 m above bed 26b; 7 m below bed 26v.
Anthraconauta kostomanovi n. sp.	19.27 m below bed 33.
Anthraconauta ex gr. A. pseudophillipsii Fed.	Roof of bed 12; between beds 12 and 13; 16.5 m above bed 25; 12.5 m below bed 26v; 6 m above bed 27b; 24.2 m above bed 28; 7 m below bed 29a; 14.45 m above bed 29; 81.5 m above bed 32; 19.27 m below bed 33; 15 m below bed 33; 43.2 m above bed 33; 50 m above bed 33; 62.55 m above bed 33; 94 m above bed 33; 106.25 m above bed 33; 66 m below bed 34; 45 m below bed 34; 44 m below bed 34; roof of bed 38.	Mass occurrence in beds: 6 m above bed 27b; 81.5 m above bed 32; 19.27 m below bed 33; 43.2 m above bed 33; 66 m below bed 34. Very small anthraconauts in beds: 16.5 m above bed 25 and 6 m above bed 27b. Unusually large anthraconauts in beds: 12.5 m below bed 26v and 7 m below bed 29a.
Anthraconauta subtenuis n. sp.	Roof of bed 12.
Anthraconauta subparallela Khalf. f. postera n.	12 m below bed 26v; 6 m above bed 27b; 37.55 m above bed 33; 45 m below bed 34; 41 m below bed 34; 25.4 m above bed 34.	Large quantity in bed: 6 m above bed 27b.
Anthraconauta subparallela Khalf. f. nano n.	16.5 m above bed 25.
Anthraconauta inflata n. sp.	21.4 m above bed 32; 31 m above bed 32.
Anthraconauta simplex Khalf.	0.8 m above bed 25; 81.5 m above bed 32; 19.27 m below bed 33; 45 m below bed 34.

under calm conditions, in the absence of swift currents or wave agitation of the water. These calm burial conditions are especially well illustrated by finds of specimens of opened shells with the valves not separated, as, for example, the specimen of Microdontella baydajevskiensis shown in Fig. 7, Pl. III. Still more indicative are specimens of microdontellas buried in life position, which is an exceedingly rare occurrence: as a general rule, with almost no exceptions, pelecypod shells in the coal-bearing deposits of Kuzbass are oriented parallel to the bedding. Such a position is, of course, post-mortem; moreover, in most cases the shells are strongly flattened by the pressure of later-deposited rocks; if they can be extracted from the rock at all, it is only in the form of completely flattened nuclei, giving no idea of the relief of the valves. In the collection from the Baidaevsky deposit there are two specimens of Microdontella subovata preserved in life position; one of them (Pl. II, Fig. 12) is very well preserved and makes it possible, more fully than was previously possible, to characterize this species, so widely distributed in the Kolchugino fauna. The exact orientation of these specimens with respect to the bedding cannot be established from the delivered cores, but this gap is filled by an observation that I was able to make in one of the exposures on the Uskat River near the village of Krasunino: in rocks with distinct bedding, a specimen of Micr. subovata was found buried in life position; it was not possible to extract this specimen, but it was established that the shell is inclined by its anterior end in such a way that the angle between its hinge margin and the bedding is about 30° (Fig. 2 in the text).

Fig. 2. Life position of the shell Microdontella subovata (Jones).

Not only specimens like those characterized above, i.e., specimens of opened shells and shells preserved in life position, indicate their burial in the place where they lived; fully preserved shells and valves in general make it possible to assert that they did not undergo any significant post-mortem transport, during which they would inevitably have experienced at least partial fragmentation. Whole shells of this kind not infrequently form mass accumulations; this applies both to anthraconautas (Pl. III, Fig. 14) and to microdontellas (Pl. III, Fig. 16). The absence of any significant current is also indicated by the disorderly arrangement of the shells in these accumulations; only in one case, in a bed lying 28 m above seam 33, was a row of shells of the species Anthraconauta pseudophillipsii found arranged in a chain, so that their long axes are subparallel (Pl. III, Fig. 15). Apparently they are situated along some water stream that had sufficient force only to turn these shells in the indicated manner, but not sufficient to transport and fragment them.

In other cases, alongside whole shells and valves, their fragments appear in greater or lesser quantity; as a rule, they are unworn and angular. A typical shell breccia in the deposits of the Erunakovskaya Formation of the Baidaevsky deposit is encountered rarely, although it has been recorded in several beds, in particular between seams 32 and 33. From consideration of the state of preservation and the conditions of burial of the shells—

species of pelecypods, it may be concluded that the fauna-bearing deposits of the Yerunakovskaya Formation of the Baidaevsky deposit were formed mainly in basins of the lake type, and only an insignificant part of them was deposited in flowing waters. In this respect they differ markedly from the rocks of the Ilyinskaya Formation of the same district: as the study of the fauna of the Ilyinskaya Formation from a number of boreholes has shown, it differs not only in composition (as was stated above), but also in its state of preservation. As a whole this fauna is less well preserved; it is true that complete shells also occur in it, and sometimes form mass accumulations (Plate III, Fig. 17), but broken shell material is very widely distributed in the Ilyinskaya Formation. At times the arrangement of shell fragments clearly indicates the presence of a strong current; thus in Fig. 18, Plate III, the surface of a core (from borehole 270) is shown, on which one can see a band of shell debris, along which regularly oriented leaves of cordaites are situated. Analysis of the fauna of the Ilyinskaya Formation leads us to the conclusion that there was great instability and fluctuation in the character of those basins in which the deposits of this formation were formed.

Below is given a description of the pelecypods of the Yerunakovskaya Formation of the Baidaevsky deposit.

On the genus Anthraconauta Pruvost.

Among the pelecypods of the Carboniferous deposits of Kuzbass, representatives of this genus are very widely distributed in all formations. Many Kuzbass anthraconauts show great similarity to the European typical species of this genus, but so far not a single species has been found that would be completely identical with any European species. Anthraconauts are also very abundant in the Kolchugino fauna, but their classification and diagnosis present considerable difficulties, since the differences between species are often far from obvious. This circumstance was the reason why D. M. Fedotov assigns all the anthraconauts of the Kolchugino fauna to only two species—Anthr. iljinskiensis and pseudophillipsii, with both species being indicated in both the Ilyinskaya and the Yerunakovskaya formations [3, 242]. This extremely simplified approach to the given group of fossils does not correspond to the actual state of affairs: within the Kolchugino anthraconauts several groups can be distinguished, each of which consists of a greater or lesser number of species. In the deposits of the Yerunakovskaya Formation of the Baidaevsky deposit alone there are 16 different species and varieties of anthraconauts, some of which, as noted above, pass over from the Ilyinskaya Formation.

Fig. 3. Anthraconauta sp. a—the anterior end of the shell; b—the lower-posterior end of the shell.

Fig. 4. Explanations in the text.

Descriptions of all these species are given below; in these descriptions the characterization of the outlines of the shell is given with such an orientation that the hinge margin occupies a horizontal position. In this case, by the anterior end of the shell is meant that part of it which lies in front ...

umboes; and by the lower-posterior end, the part of the shell cut off by a vertical drawn through the posterior point of the hinge margin (Fig. 3 in the text). The measurements adopted in the descriptions are shown in Fig. 4 in the text, namely:
D—the major diagonal, d—the minor diagonal, l—the length of the upper margin, h—the maximum height, α—the angle between the major diagonal and the hinge margin.

Among the described anthraconautids, the most important association is the assemblage of forms grouped around the species Anthraconauta pseudophillipsii; this group deserves some general consideration.

Group Anthraconauta pseudophillipsii Fed.

In addition to the species Anthr. pseudophillipsii with three varieties, this group in our fauna includes five species: A. cuneata, A. tchernyschevi, A. n. sp. cf. tchernyschevi, A. ampla, and A. propinqua. All these species are similar to one another, and there can hardly be any doubt that they are closely related; this is indicated not only by their morphological similarity, but also by certain data relating to age-related changes in their representatives (which will be discussed below). At present there is insufficient material to provide a definitive scheme of the genetic relationships among representatives of this group. However, on the basis of particular features of shell morphology and the geological distribution of the various species, one should propose, as a hypothesis, such a scheme, so that it may serve as a starting point for further investigations in this direction.

On the basis of quantitative predominance and early stratigraphic appearance, the typical Anthr. pseudophillipsii, characterized by great external similarity to the European Carboniferous Anthr. phillipsii (Will.), may be accepted as the ancestral form. From this form five different lines of development can be traced:

a) one of them, by means of complete rounding of the posterior-superior angle, in connection with which the upper margin acquires a slightly convex character, leads to the species Anthr. tchernyschevi, which is characterized by an almost regular elliptical outline of the shell;

b) the second line, through a decrease in the value of angle α while retaining considerable height of the anterior end of the shell, leads first to the variety subaequalis, from which, in turn, by changes quite analogous to those indicated for the first line of development, Anthr. n. sp. cf. A. tchernyschevi arises—a species differing from Anthr. tchernyschevi only in the smaller value of angle α, i.e., in the same character that distinguishes Anthr. pseudophillipsii var. subaequalis from the typical Anthr. pseudophillipsii;

c) the third branch is characterized (as is the second) by a decrease in angle α, but in this case the anterior end of the shell becomes lower and sharper than in the typical Anthr. pseudophillipsii. This leads to the variety angusta, and further change in the same direction leads to the species Anthr. cuneata with its characteristic wedge-shaped outlines of the shell;

d) a different character of changes characterizes the fourth line: the magnitude of angle α does not change, but the lower-posterior end of the shell becomes narrower and sharper, which characterizes the variety acuta;

e) finally, the fifth branch, by a decrease in the size of the upper-posterior angle and a considerable general widening of the shell, leads from the typical Anthr. pseudophillipsii to the species Anthr. ampla, from which, in turn, the species Anthr. propinqua is derived. It should be noted that the latter morphologically stands closer to Anthr. pseudophillipsii than does the species Anthr. ampla.

but the geological distribution of the forms being compared forces one to accept that Anthr. ampla is an ancestor of Anthr. propinqua, as has been explained in detail in the description of the latter species (see p. 123).

The relationships characterized above are presented in the form of a diagram in Fig. 5 in the text. Let us note that we have limited ourselves only to considering those species of the group Anthr. pseudophillipsii that are described in the present work; the diagram given in Fig. 5 could have been supplemented

Fig. 5. Diagram of genetic relationships among representatives of the group Anthraconauta pseudophillipsii Fed.

if we took into account certain species (for example, Anthr. trigonalis), the distribution of which is confined to the Ilyinskaya suite.

It is possible to express certain considerations also regarding the origin of the group Anthr. pseudophillipsii; observations on the character

Fig. 6. Anthraconauta propinqua n. sp. Greatly enlarged; character of the growth lines of the shell.

Fig. 7. Anthraconauta ampla n. sp. Greatly enlarged; character of the growth lines of the shell.

of the growth lines of the shell show that, in the young stages, representatives of all the species of this group were, first, similar to one another and, second, differed substantially from adult specimens, namely: the young shells were developed in length, and not along the main diagonal; they were more similar to the anthraconautas of the Anthr. subparallela group (see p. 125), as can be seen in Figs. 6 and 7 in the text, where the shells of the species Anthr. propinqua and Anthr. ampla are shown schematically and greatly enlarged. This is also directly confirmed by the young individuals of the indicated species available in our collection (see, for example, Fig. 3 on Pl. II). We are fully justified in concluding that the group considered here originated from anthraconautas with a very small value of the angle \(\alpha\) and with a subparallel position of the ventral margin in relation to the hinge—

i.e., from the anthraconaut, outwardly close to the genus Anthraconauta Tr. et W., as, for example, the already mentioned Anthraconauta subparallela.

The same observations make it possible to establish how, in the process of uneven growth of the shell, species differences arose among the components of this group. This is shown schematically in Fig. 8 in the text; if, for consideration of this process, we take as the starting point the generalized type of young shell indicated above, then, tracing the successive growth lines, we can establish three principal cases:

Fig. 8. Scheme of shell growth: a—Anthr. propinqua; b—Anthr. ampla; c—Anthr. pseudophillipsii

a) if shell growth occurs predominantly along the main diagonal, i.e., in the direction of the lower-posterior margin, there arises a shell type characteristic of the species Anthr. propinqua;

b) if the shell grows predominantly along the hinge margin backward and along the minor diagonal (in the direction of the ventral margin), there arises a shell characteristic of the species Anthr. ampla;

c) the third case is that of predominant shell growth along the main and minor diagonals, which leads to a shell type typical of Anthr. pseudophillipsii.

These three modes of shell growth are also shown schematically in Fig. 8; the arrows indicate the directions of predominant growth, although, of course, the shell increases in all directions.

On the species Anthraconauta pseudophillipsii Fed.

In 1937 D. M. Fedotov described and figured, under the name Anthr. pseudophillipsii n. sp., a large group of specimens originating mainly from the Erunakovskaya suite, giving the following very schematic diagnosis for this species: “Shell small, short, its length only about twice its height, weakly convex. Form highly variable and outwardly similar to the western European A. phillipsi (Williamson)” [2, 37]. A diagnosis formulated in this way allowed D. M. Fedotov to assign to A. pseudophillipsii very different shells, as can be seen from the photoplates presented in the cited work. This is also noted by the author of this species in the description of the latter; namely, D. M. Fedotov distinguishes three groups among the specimens he assigned to Anthr. pseudophillipsii:

1—shell “with a clearly projecting anterior part, with a sharp difference between the short, narrow anterior part and the long, expanded, posterior part of the shell not tapering at the end” (cited work, p. 37);

2—very narrow, long shells, with smoothed umbones, with a weakly expressed anterior part;

3—comparatively high and relatively short shells, “with a long, straight hinge margin, with weakly expressed umbones and an almost unexpressed, shortened anterior part” (ibid.).

It should be added that these three groups do not exhaust the diversity of anthraconauts assigned by D. M. Fedotov to the given species; thus, the specimen figured in Pl. VI, fig. 17, differs sharply from all the others and belongs to the species Anthr. simplex Khalf. [4, 67]. Such specimens, of course, must be separated from the species Anthr. pseudophillipsii; but even after this the given species remains highly variable, and it seems useful to me to refine and characterize more fully the variability of its representatives.

As typical specimens for this species, it seems to me, one should choose those which are morphologically closest to the European Anthr. phillipsii; such should be regarded as the specimens of the third group (see above): these are shells developed along the main diagonal, with an angle $\alpha$ of about 40–45°, with a long but rounded lower posterior end. Insofar as I can judge from the material available to me, the variability within the species Anthr. pseudophillipsii from this typical form proceeds in three directions:

a) in some specimens the lower posterior end is strongly narrowed and pointed; such are the specimens of the first group (see above), for which the name var. acuta has been proposed;

b) in other specimens a decrease of the angle $\alpha$ is observed; the shell lengthens and becomes narrower (the second group—see above); this leads to the variety angusta;

c) in the third case analogous changes (decrease of the angle, elongation of the shell) are accompanied by widening of the anterior end, which leads to the variety subaequalis.

In addition to these varieties, I have specimens of typical Anthr. pseudophillipsii that show changes leading to the development of characters characteristic of the species Anthr. ampla (see below); these changes consist in the shell becoming broader owing to the increasing convexity of the ventral margin and to the development of that part of the shell which adjoins the upper posterior angle. This permits me to suggest that the species Anthr. ampla derives from Anthr. pseudophillipsii.

Below is given a description of Anthr. pseudophillipsii and its varieties from the Erunakovskaya Formation of the Baidaevsky deposit.

Anthraconauta pseudophillipsii Fed. f. typ.

Pl. I, figs. 1–8.

Anthr. pseudophillipsii (in part), Fedotov, 1937, p. 37, Pl. VII, fig. 7.
Anthr. pseudophillipsii (in part), Fedotov, 1938, p. 236, Pl. V, figs. 8, 9.
Anthr. pseudophillipsii Khalf., 1950, Pl. XIII, figs. 1, 2.

Description. The shell reaches large size and is developed along the main diagonal: angle $\alpha$ about 40–45°. The umbones are small, sharp, noticeably raised above the hinge line; they are strongly shifted toward the anterior margin, owing to which the anterior end is short but quite distinctly developed. The length of the upper margin exceeds half the length of the main diagonal, reaching in some specimens $2/3$ of the latter. The posterior upper angle is completely rounded; the convex posterior margin is directed at a very obtuse angle to the hinge margin. The shell widens strongly posteriorly; the greatest height exceeds half, but does not reach $2/3$, of the length of the main diagonal. The lower posterior end is long and broad; the ventral margin is evenly convex, in some large specimens slightly straightened in the middle part. The shell is moderately broad: the minor diagonal is always somewhat greater than half the main diagonal. Sculpture ordinary.

Dimensions:

D	d	l	h	α	d/D	l/D	h/D	No.	specimens
9	5	6	5		0.59	0.66	0.59	396	(young specimen)
22	12.5	12.5	13	40°	0.57	0.57	0.60	75—1
~22	12	12	12	—	0.55	0.55	0.55	120—1
24	~13	~14	15	~40°	~0.54	~0.58	0.62	132
27	14.5	15.5	16.5	—	0.54	0.57	0.61	404
28	16	18	17	43°	0.57	0.64	0.61	390
17.5	11	12	10.5	35°	0.63	0.70	0.60	149	transition to the species Anthr. ampla n. sp.
23.5	15	14.5	16	35°	0.64	0.62	0.68	48—1	transition to the species Anthr. ampla n. sp.
17	9	10	9	30°	0.53	0.59	0.53	405—1	(transition to the variety angusta)
17	9	9	11	30°	0.53	0.53	0.65	18—1	(transition to the variety acuta)

Comparisons. The differences of the varieties acuta, angusta, and subaequalis are indicated in the descriptions of the latter. The closest species is Anthr. supraphillipsii Khalf. [4, 49] from the lower parts of the Erunakovskaya suite on the Tom River below the village of Kazankova; this species differs by a shorter hinge margin, an unrounded posterior–upper angle, and a straight posterior margin in its upper part. Anthr. ampla is sharply distinguished by a broader shell of subtriangular outline, a longer hinge margin, and the small size of the posterior–upper angle; but, as already noted above, there are specimens (spec. 149, pl. I, fig. 6, and spec. 48—1, pl. I, fig. 5) showing a transition from Anthr. pseudophillipsii typ. to Anthr. ampla: in them the ratios of the minor diagonal, hinge margin, and shell height to the principal diagonal are greater than in typical Anthr. pseudophillipsii, and approach those of representatives of the species Anthr. ampla.

Distribution. Anthr. pseudophillipsii typ. is distributed in both the Ilyinskaya and the Erunakovskaya suites. In the Baidaevsky deposit this form in some beds forms mass accumulations, for example 28 m above seam 33 and 37.5 m above the same seam, and also 1.5 m above seam 20. Specimens of Anthr. pseudophillipsii typ. have been encountered between seams 12 and 13, in the roofs of seams 14, 22, and 25, 19 m below and 37.5 m above seam 33; specimens transitional to the species Anthr. ampla were obtained from beds 12.5 m above seam 26 b, 19.5 m below seam 33, and 45 m below seam 34. The presence of this form is very probable also in other faunistic beds containing anthraconauts of poor preservation.

Anthraconauta pseudophillipsii Fed. var. acuta Khalf.

Pl. I, figs. 12–13.

Anthraconauta pseudophillipsii Fedotov (in part), 1937, p. 37, pl. VI, figs. 15, 16, 19.
Anthraconauta pseudophillipsii var. acuta Khalfin, 1949, pl. XIII, fig. 4.

Differences from the typical form. This variety differs from the typical form by a longer, narrower, and more pointed lower–posterior end; because of this the shell appears somewhat higher, and the posterior and ventral margins are less convex, in some specimens almost straight.

Dimensions:

D	d	l	h	α	d/D	l/D	h/D	Specimen no.
10.5	6.5	5.5	7	38°	0.62	0.52	0.67	61
12	6.5	7	8	33°	0.54	0.58	0.67	18—3
12	6.5	7	7	33°	0.54	0.58	0.58	63

Remarks. This variety occurs rarely; moreover, I never once found representatives of it that attained the size of adult shells of the typical Anthr. pseudophillipsii. The natural question arises whether the specimens that I assign to this variety are not juvenile specimens of Anthr. pseudophillipsii typ. This question must be answered in the negative, since tracing the growth lines shows that in the early stages of development representatives of the species Anthr. pseudophillipsii had a shell only slightly widening backward, with a short and rounded lower-posterior end, i.e., possessed precisely features opposite to those characteristic of the variety acuta.

The features of this variety are expressed most typically by specimen 18—3 (pl. I, fig. 12); specimen 63 (pl. I, fig. 13) is distinguished by an umbo brought extremely close to the anterior margin.

Distribution. This variety, like the typical form, is distributed in both the Ilyinskaya and the Erunakovskaya suites. From the Baidaevsky deposit only three specimens have been obtained, which come from the following faunistic beds of the Baidaevsky horizon: 18 m above seam 25; 12 m below seam 26v (in this same bed larger anthraconautas were also encountered, displaying transitional characters between typical Anthr. pseudophillipsii and the variety acuta: see pl. I, figs. 7, 8); 7 m above seam 26v.

Anthraconauta pseudophillipsii Fed. n. var. angusta.

Pl. I, figs. 9—11.

Anthraconauta pseudophillipsii Fedotov (in part), 1937, p. 37, pl. VII, fig. 6.

Holotype: 156—Ara, Baidaevsky deposit, Erunakovskaya suite, Abashevsky horizon; 8.5 m below seam 23.

Differences from the typical form. A decrease in angle α, with a parallel increase in the posterior-upper angle, gives the shell more elongated outlines; at the same time, however, the shell widens backward considerably, since its anterior end is very weakly developed—short and narrow. On the whole the shell is considerably narrower and lower than in Anthr. pseudophillipsii typ.: the small diagonal is equal to or less than half the main diagonal.

Dimensions:

D	d	l	h	α	d/D	l/D	h/D	Specimen no.
16	8	11	8	32°	0.50	0.69	0.50	156 (holotype)
17	8	11	8	20°	0.47	0.65	0.47	405—2
20	10	12	10	—	0.50	0.60	0.50	152—1
31	14.5	18	16	30°	0.47	0.58	0.52	417

Distribution. In the collections from the Baidaevsky deposit that I have, this variety belongs to the narrow stratigraphic interval of seams 23—24—25, coming from the following faunistic beds: 8.5 m below seam 23; 28.75 m above seam 24; the roof of seam 25 (from here also come specimens transitional from the typical form to the variety angusta—see Table I, fig. 4). The specimen of D. M. Fedotov cited in the synonymy comes from the Erunakovskaya suite near the village of Sokolovaya (roof of seam 666).

Anthraconauta pseudophillipsii Fed. var. subaequalis Khalf.

Table I, figs. 14—16.

Anthraconauta iljinskiensis (in part) Fedotov, 1937, p. 35, pl. VI, fig. 12.
Anthaconauta pseudophillipsii (in part) Fedotov, 1938, p. 236, pl. V, fig. 7.
Anthraconauta pseudophillipsii var. subaequalis Khalfin, 1950, pl. XIII, fig. 3.

Differences from the typical form. Like the variety angusta, the present variety differs from the typical form in the outline of the shell, which is more elongated, narrower, and less high, which is connected with a reduction of the angle α. But, unlike the variety angusta, in the present form the shell widens only slightly backward, since its anterior end is relatively very high and the direction of the ventral margin forms a very acute angle with the hinge margin.

Dimensions:

D	d	l	h	α	d/D	l/D	h/D	No. of specimens
15	8	8	8		0.53	0.53	0.53	33—1
15.5	7.5	10	7.5	25°	0.48	0.64	0.48	43—1
17	9.5	11	9.5	30°	0.56	0.65	0.56	45—1
34	18	18	18	33°	0.53	0.53	0.53	140

Comparisons. The differences from the variety angusta are indicated above. At first glance the present variety shows great similarity to Anthr. propinqua (compare fig. 16 on Table I and fig. 1, Table II); however, there is a whole series of differences between them, namely: in the form described here the umbones are more distant from the anterior margin, the postero-dorsal angle is more obtuse, the postero-ventral end is longer, and the anterior end is considerably higher, owing to which the subtriangular outlines of the shell characteristic of Anthr. propinqua are not expressed in the variety described here.

Distribution. Like the typical form, this variety is distributed in the Ilyinskaya and Erunakovskaya suites. In the Baidaevsky deposit this variety is distributed as follows: one unusually large specimen (about 3.5 cm in length) was obtained from the bed 6.7 m below seam 16 (Table I, fig. 16), and then this variety is encountered in large numbers much higher up (seam 33), where it is represented by specimens twice as small (Table I, figs. 14, 15); here it was found in beds: 15 m below, 55 m above, 55.35 m above, and 56 m above seam 33.

Anthraconauta cuneata n. sp.

Table I, fig. 17.

Holotype: 145—Ac, Baidaevsky deposit, Erunakovskaya suite, Abashevsky horizon, 25 m above seam 22.

Diagnosis. A small shell of wedge-shaped outline: the anterior end is very short and narrow; posteriorly the shell expands strongly and uniformly—the straight ventral margin is directed at an angle of 35° to the hinge margin. The posterior margin is convex; the upper-posterior angle is not expressed at all (cut off). The lower-posterior end is broadly rounded, not drawn out. The umbones are situated close to the very anterior margin (almost terminal). The hinge margin is long—exceeding \(^{2}/_{3}\) of the length of the main diagonal. The greatest height of the shell, passing through the posterior end of the hinge margin, is somewhat less than the length of the hinge margin. The surface of the valves is covered with concentric lines, among which a few are coarser than the rest.

Dimensions:

\(D\)	\(d\)	\(l\)	\(h\)	\(x\)	\(\dfrac{d}{D}\)	\(\dfrac{l}{D}\)	\(\dfrac{h}{D}\)
14.5	8	10	8.5	32	0.55	0.69	0.59

Comparisons. The greatest similarity to the present species is shown by Anthr. sphenoidalis Khalf. [5], widely distributed in the deposits of the Ilyinskaya suite near the village of Kazankova; the similarity consists in the general wedge-shaped outline of the shell in both species. Anthr. sphenoidalis differs in its convex ventral margin and always dwarf dimensions: the largest representatives of this species are half the size of Anthr. cuneata; ordinarily the shell in Anthr. sphenoidalis is only 4–5 mm long.

I believe that Anthr. cuneata is closely related to Anthr. pseudophillipsii var. angusta, from which it probably originated. Both these forms have been found in the upper part of the Abashevo horizon. The distinguishing features of Anthr. cuneata are the wedge-shaped outlines of the shell, which is moreover broader and higher, the straight ventral margin, and umbones closer to the anterior margin.

Distribution. A rare form; I have only one specimen of this species—the holotype, whose stratigraphic position was indicated above.

Anthraconauta tchernychevi Khalf.

Table II, figs. 8–9.

Anthraconauta (Anthracomya) tchernychevi Khalf., 1950, p. 52, pl. I, figs. 4–6.
Anthraconauta tchernychevi Khalf., 1950, pl. XIII, figs. 11, 12; pl. XIV, fig. 1.

This species is characterized by an almost regular, obliquely elliptical outline of the shell: the junction of the posterior and dorsal margins is so smooth and gradual that it cuts off not only the upper-posterior angle, but almost the entire upper-posterior field; the hinge margin becomes slightly convex, and it becomes very difficult to measure its length. Anthr. tchernychevi occurs both in the Ilyinskaya and in the Erunakovskaya suites. In the collection described, representatives of this species come from the following beds: 30 m above seam 25; 18.65 m above seam 26v; 15 m below seam 33. Dimensions of these specimens:

\(D\)	\(d\)	\(l\)	\(h\)	\(x\)	\(\dfrac{d}{D}\)	\(\dfrac{l}{D}\)	\(\dfrac{h}{D}\)	Specimen no.
9	6.5	~6	6.5	40	0.72–0.67	0.72	0.72	134
~16	11	—	12	—	0.69	—	0.75	122–1
19.5	12	12	13	40	0.62	0.62	0.67	46

Anthraconauta n. sp., cf. A. tchernychevi Khalf.

Table II, figs. 10—11.

Alongside typical Anthr. tchernychevi, both in the Ilyinskaya and in the Yerunakovo formations there occur anthraconautas which also have subelliptical shell outlines, but differ from Anthr. tchernychevi in the smaller value of angle $\alpha$ (about 30): in these anthraconautas the shell is developed more in length than in the diagonal direction; often (though not always) it is narrower, and the umbones are farther removed from the anterior margin than in Anthr. tchernychevi. Comparing the shells of these two forms with one another, and also with the species Anthr. pseudophillipsii and its varieties, I arrive at the following conclusions:

a) the two compared groups of shells with elliptical outlines represent two different species that are not directly connected with one another;

b) both species originated in parallel from anthraconautas of the Anthr. pseudophillipsii group, but whereas Anthr. tchernychevi derives from typical Anthr. pseudophillipsii, Anthr. sp. cf. A. tchernychevi derives from the variety subaequalis; this is indicated by the development of the shell in length (a smaller value of $\alpha$) and the greater development of the anterior end in Anthr. n. sp. cf. tchernychevi;

c) both species derive from anthraconautas of the Anthr. pseudophillipsii group by the cutting off of the postero-dorsal field by a broad rounding, with which are associated an increase in the convexity of the posterior margin and the acquisition of some convexity by the hinge margin.

I intend to verify the conclusions set forth above on additionally collected material; therefore, for the present I refrain from assigning a separate name to the species described.

The dimensions of shells of this species are sometimes very considerable: one incompletely preserved specimen (specimen 179—1) reaches 3 cm in length; two other specimens have the following dimensions:

D	d	l	d	$\alpha$	$\dfrac{d}{D}$	$\dfrac{l}{D}$	$\dfrac{h}{D}$	Specimen no.
19	11	—	12	—	0.58	—	0.63	131—2 (holotype)
24	15	~15	16	33°	0.65	0.65	0.67	73

Specimens of this species were obtained from the following faunistic beds: 10 m, 11.25 and 20.8 m above seam 266; 19.5 m below seam 33.

Anthraconauta ampla n. sp.

Table I, figs. 21—22.

Holotype: 176-Aa, Baidaevsky deposit, Yerunakovo formation, Baidaevsky horizon, 29.5 m above seam 266.

Diagnosis. A large, broad shell, in outline approaching a right triangle with strongly rounded corners. The upper margin is long (about 3/4 of the length of the main diagonal); the posterior margin weakly convex; the ventral margin in its middle part straight or even slightly concave. The postero-dorsal angle is completely rounded; overall, the posterior margin with the hinge forms an angle of 95—105°. The postero-ventral end is cut off by a broad rounding, along which it very smoothly con—

meet at the posterior and ventral margins; in the holotype the latter is directed, with respect to the hinge margin, at an angle of \(25^\circ\). The anterior end is rounded, much narrower than the posterior end (the shell widens very strongly backward). The weakly developed umbones are removed from the anterior margin by a distance of about \(1/4\) of the length of the main diagonal. The shell is high and broad: the greatest height, passing through the posterior point of the hinge margin and equal to the minor diagonal, amounts to \(2/3\)—\(3/4\) of the length of the main diagonal. The surface of the valves is covered with the dense, regular concentric growth lines usual for the genus Anthraconauta.

Measurements:

D	d	l	h	\(x\)	\(\frac{d}{D}\)	\(\frac{l}{D}\)	\(\frac{h}{D}\)	No. of specimens
12	6.5	8	6.5	—	0.54	0.67	0.54	384 (young specimen)
22	14	16	14	—	0.64	0.73	0.64	413
27	17	19	17	\(28^\circ\)	0.63	0.70*	0.63	176 (holotype)
29	21	21	21	\(\sim 35^\circ\)	0.72	0.72	0.72	133

Comparisons. The distinguishing characters of this species are the very small size of the upper-posterior angle (more precisely, the angle between the directions of the posterior and hinge margins, since the upper-posterior angle itself is cut off by a broad rounding) and the large value of the minor diagonal relative to the main one, as a result of which the shell proves to be very broad. These features (as well as the umbones being more distant from the anterior margin) distinguish this species from the related species Anthr. propinqua. These differences quite definitely separate adult representatives of the compared species, whereas their young specimens are difficult to distinguish (by the position of the umbones). The emergence, in the course of growth, of diagnostic differences between Anthr. ampla and Anthr. propinqua is explained in detail in the description of the latter species.

Distribution. Anthr. ampla occurs through all horizons of the Erunakovo suite of the Baidaevsky deposit, beginning with the lower part of the Abashevo; its first representatives appear in the roof of seam 12, and higher up they have been obtained from the following beds: roof of seam 14; 18 m above seam 25; 29.5 m above seam 266; 19.5 m below seam 33; 37.5 m above seam 33; 66 m below seam 34; in the faunistic beds situated 30 m above seam 25 and 45 m below seam 34 there are incomplete specimens that apparently belong to this same species. It is the guiding form for the Erunakovo suite (absent in the Ilyinskaya).

Anthraconauta propinqua n. sp.

Plate II, figs. 1—3.

Holotype: 180-Ar, Baidaevsky deposit, Erunakovo suite, Baidaevsky horizon; 16 m above seam 266.

Diagnosis. A large shell, strongly widening backward, in outline approaching an oblique-angled triangle with strongly rounded corners. In adult specimens the upper margin reaches \(2/3\) of the length of the main diagonal. The upper posterior angle is strongly rounded. The convex posterior margin is directed at an angle of about \(120^\circ\) to the hinge margin. The lower-posterior end is broadly rounded. The ventral margin over the greater part of its extent is straight or slightly concave; in the holotype it is directed at an angle of \(22^\circ\) to the hinge margin. The anterior end of the shell is very weakly ...

developed; narrow and very short because the beaks are very strongly shifted toward the anterior margin. The shell is low and rather narrow: the greatest height, passing through the posterior end of the hinge margin, usually equal to the minor diagonal, does not reach \(2/3\) of the length of the main diagonal. The convexity of the valves is considerable; the diagonal elevation is rounded, becoming smoothed out within the lower-posterior end of the shell. The surface of the valves is covered with very dense and fine concentric lines.

Dimensions:

D	d	l	h	\(\alpha\)	\(\dfrac{d}{D}\)	\(\dfrac{l}{D}\)	\(\dfrac{h}{D}\)	No. of specimens
16	9,5	12	9,5	—	0,60	0,75	0,60	27 (young specimen)
25	15	16	15	40°	0,60	0,64	0,60	21
26	14	17	14,5	38°	0,54	0,65	0,56	180 (holotype)
29	17	—	16	—	0,59	—	0,55	76

Remarks. The diagnosis given above characterizes adult representatives of the species Anthr. propinqua; the shells of its young representatives, as specimen 27 shows (Table II, fig. 3), exhibit substantial differences, due primarily to the fact that in them the upper posterior angle is considerably smaller—close to a right angle. Because of this they show greater similarity to the species Anthr. ampla than to adult Anthr. propinqua. Comparing young shells of Anthr. ampla (specimen 384) and Anthr. propinqua (specimen 27), and tracing the character of growth features on the holotypes and other specimens, I arrived at the following conclusions:

a) if adult individuals of these two forms are easily distinguishable (see the description of Anthr. ampla), then younger shells show almost complete identity: the difference consists only in the position of the beaks, which in Anthr. propinqua are closer to the anterior margin than in Anthr. ampla;

b) the diagnostic differences between these species arise as a result of differences in the directions of shell growth: in Anthr. propinqua the shell grows chiefly along the main diagonal, toward the lower-posterior end; because of this the value of the upper-posterior angle increases, the lower-posterior end lengthens, the ratio of the upper margin to the main diagonal decreases, while the ratio between the minor and main diagonals remains without significant change (i.e., the shell remains rather narrow even in the adult state); in Anthr. ampla the shell grows more uniformly in two directions: backward along the hinge margin and along the minor diagonal toward the ventral margin; because of this the value of the posterior-upper angle remains without substantial change, the lower-posterior end remains short, the ratio of the length of the upper margin to the main diagonal changes hardly at all, but the ratio of the minor diagonal to the main increases (i.e., the shell becomes broader);

c) the great similarity in the characters of the young shells of the species being compared must indicate their close genetic relationships, while the character of age-related changes and the stratigraphic distribution of Anthr. ampla and Anthr. propinqua permit the conclusion that the latter is a descendant of the former.

Comparisons. Morphologically it is very close to Anthr. trigonalis Khal. [4, 55]; this similarity is so considerable that initially I was inclined to assign the specimens for which the species Anthr. propinqua was established to the species Anthr. trigonalis. But the clarification of the age-related changes and genetic relationships of Anthr. propinqua to Anthr. ampla compels me to consider

similarity between Anthr. trigonalis and Anthr. propinqua as the result of convergence. This is also confirmed by the difference in the stratigraphic position of the forms being compared: Anthr. trigonalis is distributed in the Ilyinka Suite and in the lower parts of the Erunakovo Suite, whereas Anthr. propinqua occurs only in the upper horizons of the latter. Therefore the morphological differences (however small they may be) between these two species acquire greater significance; these differences are as follows: the shell in Anthr. trigonalis is narrower—the small diagonal is somewhat less than half the main diagonal, whereas in Anthr. propinqua the small diagonal is always noticeably greater than half the main diagonal; the posterior margin in Anthr. propinqua is more convex, and the postero-ventral end is broader and shorter.

The differences from Anthr. pseudophillipsii are indicated in the description of the latter.

Distribution. Anthr. propinqua is confined to a narrow stratigraphic interval, belonging mainly to the very lowest parts of the Baidaevsky horizon; representatives of this species were obtained from the following beds: 19 m and 15 m below seam 26a; 16 m above seam 26b; 7 m below seam 26v.

Anthraconauta kostomanovi n. sp.

Table II, fig. 5.

Holotype: 119-Ak, Baidaevsky deposit, Erunakovo Suite, Baidaevsky horizon, 19.27 m below seam 33.

Diagnosis. Shell of an outline unusual for the genus Anthraconauta: the slightly convex ventral margin, over most of its extent, is almost parallel to the hinge margin—the shell scarcely widens posteriorly, although its maximum height nevertheless passes through the posterior end of the hinge margin. The length of the latter is \(2/3\) of the main diagonal; the postero-dorsal angle, though strongly rounded, is clearly expressed, about 135°. The posterior margin is uniformly convex; the postero-ventral end is short, broadly rounded. The ventral margin only within the anterior third of the shell length rises rather steeply upward, passing quite imperceptibly into the uniformly rounded anterior margin. The anterior end of the shell is short, but relatively very high. The maximum height of the shell is about \(2/3\) of the main diagonal. The beaks are removed from the anterior margin by a distance of \(1/5\) of the shell length. The surface of the valves is covered with very fine and densely arranged concentric lines and with comparatively few, widely spaced, sharp growth marks.

Dimensions:

\(D\)	\(d\)	\(l\)	\(h\)	\(d/D\)	\(l/D\)	\(h/D\)
26	16	16	17.5	0.62	0.62	0.67

Remarks. This original form is represented in our collection by only a single large specimen, which does not permit determination of the character of the relief and of the beaks (the specimen is strongly flattened), but this sample possesses such obvious differences from all known anthraconauts that the establishment of a new species for it is necessary. The subparallelism of the ventral and hinge margins, owing to which the shell expands very weakly posteriorly, distinguishes this species from most anthraconauts and brings it closer to some representatives of the genus Anthraconia with a weakly developed anterior end of the shell.

Comparisons. The most important distinguishing features of this species are the subparallelism of the ventral and hinge margins and the very high shell \((h = 0.67D)\). In the Erunakovskaya Formation similar anthraconautids are absent; Anthr. subparallela [4, 60] has a less high shell \((h\) about \(0.5D)\) and usually a longer hinge margin. The greatest resemblance is shown by Anthr. vulgaris Khalf. [5] from the second Alykayevskaya fauna, which differs in its unrounded posterodorsal angle, straight posterior margin, and other details in the outlines of the shell. These differences, in connection with the great difference in the stratigraphic positions of these two species, rule out the possibility of identifying them.

Distribution. The form is very rare: the species is established from a single specimen (holotype), the locality of which is indicated above.

Anthraconauta subtenuis n. sp.

Table II, fig. 6.

Holotype: 422-As, Baidaevsky deposit, Erunakovskaya Formation, Abashevsky horizon, roof of bed 12.

Diagnosis. Shell of rather large size, very slightly widening posteriorly (angle \(\alpha\) about \(20^\circ\)), long and very narrow: the minor diagonal is considerably less than half the main diagonal. The hinge margin is very long (up to \(3/4\) of the main diagonal), joining with the convex posterior margin entirely smoothly, without formation of a posterodorsal angle. The ventral margin is straight, forming with the hinge margin an angle of about \(15^\circ\). The umbones are strongly shifted toward the anterior margin. The sculpture consists of very fine and dense concentric lines.

Dimensions:

\(D\)	\(d\)	\(l\)	\(h\)	\(\alpha\)	\(\dfrac{d}{D}\)	\(\dfrac{l}{D}\)	\(\dfrac{h}{D}\)
21	9	16	10	\(22^\circ\)	0.43	0.76	0.47

Comparisons. The extremely narrow and long shell with a very long hinge margin readily distinguishes this species from all other anthraconautids of the Ilyinskaya and Erunakovskaya formations.

Distribution. Anthr. subtenuis characterizes the lower part of the Abashevsky horizon; representatives of this species were encountered in the roof of bed 12 in two faunistic layers at a distance of 6 m from one another.

Anthraconauta subparallela Khalf. postera nova.

Table II, fig. 7.

Holotype: 127—A sp, Baidaevsky deposit, Erunakovskaya Formation, Esaulsky horizon, approximately 45 m below bed 34.

Diagnosis. A small, strongly oblique shell that widens very weakly posteriorly; the greatest height, passing through the posterior end of the hinge margin, is half the main diagonal. The hinge margin is equal to approximately \(2/3\) of the shell length; the posterodorsal angle is very obtuse and strongly rounded, the posterior margin convex. The ventral margin is also uniformly convex, directed at a very acute angle to the hinge margin. The posteroventral end is short, rounded; the anterior end narrow and very short. The massive umbones are noticeably raised above the hinge line, very close to the anterior margin, but in front of the umbones there nevertheless re-

The chetlium is expressed by a segment of the hinge margin lying at the same level as the upper margin behind the beaks. The diagonal elevation is broad, flatly rounded, and becomes smoothed out within the lower-posterior end of the shell. The sculpture consists of regular, in part rather sharp concentric lines.

Dimensions:

	D	d	l	h	z	d/D	l/D	h/D	Specimen no.
	12	6	8	6		0.50	0.67	0.50	127 (holotype)
	14.5	6	10	6		0.41	0.69	0.41	75—2

Comparisons. The typical Anthr. subparallela [4, 60], widely distributed in the lower part of the Erunakovo Series below the Kazankovo bed along the Tom’ River, differs in having a considerably longer hinge margin (equal to 4/5 of the shell length), a smaller posterior-upper angle, and a weakly convex, even straight in its middle part, ventral margin.

Distribution. One damaged specimen comes from the bed 12 m below seam 26v. Numerous representatives of this form, together with small Anthraconauta ex gr. A. pseudophillipsii, are present in the bed 6 m above seam 27b; individual specimens have been found in beds occurring 37.5 m above seam 33, 45 m and 41 m below seam 34, and 25.5 m above seam 34.

Anthraconauta subparallela Khalf. forma nana nova.

Holotype: 173-Asn, Baidaevsky deposit, Erunakovo Series, Abashevo horizon, 16.5 m above seam 25.

This form differs from the form postera described above only in the dwarf size of its representatives, which are only 5–6 mm in length. They were found in one bed, occurring 16.5 m above seam 25; in most cases the specimens of this form are imperfectly preserved, although they are fairly numerous. Together with them in this bed there are also other anthraconautids, close in shell outline to Anthr. pseudophillipsii, but possessing equally small dimensions. It must be supposed that during the deposition of this bed there existed some unfavorable conditions that had an inhibiting effect on the anthraconautids.

Anthraconauta inflata n. sp.

Table II, fig. 4.

Holotype: 199-Ai, Baidaevsky deposit, Erunakovo Series, Baidaevsky horizon, 21.5 m above seam 32.

Diagnosis. A very inflated shell, weakly widening posteriorly: the ventral margin is subparallel to the hinge margin, which makes up 2/3 of the length of the main diagonal and passes quite smoothly, without forming a posterior-upper angle, into the posterior margin. The latter is convex; in its middle and lower parts it is directed almost perpendicular to the hinge margin and, smoothly rounding, without forming an elongated lower-posterior end, passes into a weakly and evenly convex ventral margin. The weakly developed beaks are very strongly drawn close to the anterior margin—the anterior end is short but high, bounded by an evenly convex margin. The greatest height of the shell passes through the posterior end of the hinge margin and is equal to the length of the po-

the latter. The valves are strongly and almost uniformly inflated; the diagonal elevation is rounded, not separated from the rest of the valve surface. The surface of the latter, from the diagonal elevation, descends very smoothly and gradually toward the ventral margin and more steeply toward the hinge margin. The sculpture consists of very fine and dense concentric lines.

Dimensions of the holotype:

D	d	l	h	α	d/D	l/D	h/D
20	12	13	13	32	0.60	0.65	0.65

Comparisons. Anthr. inflata differs from all other anthraconautids by the extraordinarily convex valves; this character distinguishes the present species also from the closest in outline, Anthr. subparallela Khalf., and from the above-described variety postera of the latter species.

Remarks. The holotype of this species is a well-preserved internal mold of a right valve. Another specimen is represented by impressions of both valves; the impressions are somewhat incomplete, but they make it possible to establish the strong inflation of the shell and the same character of its outline as in the holotype.

Distribution. Both specimens were found above seam 32: one at a distance of 21.5 m, the other at a distance of 31 m from the roof of this seam.

Anthraconauta simplex (Khalf.)

Table I, figs. 18—19.

Anthracomya minima (partim) Jones, 1901, p. 433, pl. XVI, figs. 3, 4.
Anthraconauta pseudophillipsii (partim) Fedotov, 1937, p. 37, pl. VI, fig. 17.
„ iljinskiensis (partim) Fedotov, 1938, p. 235, pl. V, fig. 5.
Anthracomya tomiensis, Khalfina, 1939, p. 65, pl. III, fig. 3.
„ simplex „ 1939, p. 67, pl. III, figs. 4, 6.
Anthracomya simplex „ 1950, pl. XV, figs. 1, 2.

This species, whose most important distinguishing character is the fact that the greatest height of the shell passes not through the posterior end of the hinge margin but approximately through the middle of the latter, occurs both in the Ilyinskaya and in the Erunakovo suites. In the Baidaevsky deposit, its few representatives have been found in the Abashevsky and Baidaevsky horizons, namely: in the roof of seam 25; at a distance of 81.5 m from the roof of seam 32; at a distance of 19 m below the floor of seam 33; at a distance of 45 m below seam 34. The dimensions of the Baidaevsky representatives of this species are as follows:

D	d	l	h	α	d/D	l/D	h/D	Specimen no.
11	7	7	6	25	0.64	0.64	0.55	120—2
12	7	8	7	26	0.58	0.67	0.58	159—1

Anthraconauta lata (Khalf.)

Table I, fig. 20.

Anthraconauta lata Khalfina, 1939, p. 68, pl. III, figs. 5, 7.
Anthraconauta lata Khalfina, 1950, pl. XV, figs. 4, 5.

This very characteristic species, distinguished from all other Kolchugino anthraconautids by its unusually high and short shell, was described by me from deposits of the Ilyinskaya suite below the village of Kazankova.

Materials from the Baidaevsky deposit show that it is also distributed in the Erunakovskaya suite; moreover, its Erunakovskaya representatives have larger dimensions, and their posterior-upper angle is somewhat greater (about 130°), while the hinge margin is relatively shorter. The dimensions of our specimens of Anthr. lata are as follows:

D	d	l	h	x	d/D	l/D	h/D	Specimen No.
16	12.5	10	12.5	45°	0.78	0.62	0.78	128-1
17	12.5	11	12.5	45°	0.74	0.65	0.74	155-1
18	13.5	10	14	45°	0.75	0.56	0.78	410-1

These specimens occupy the following stratigraphic position: roof of seam 24, 43 m above seam 33, 45 m below seam 34.

On the genera Microdonta Khalf., Microdontella Leb. and Abiella Rag.

The fauna of Paleozoic pelecypods of the Kuzbass is one of the most difficult to study among the fossil faunas of Western Siberia; this is due, given its quite endemic composition, primarily to the fact that the nature of preservation of the fossil pelecypods of the Kuzbass makes it impossible in the overwhelming majority of cases to study the internal structure (in particular, the structure of the hinge) [5]. The greatest difficulties, in terms of classification and diagnosis, are presented by the group of Kolchugin pelecypods, which includes the three genera named above. Most of the pelecypods of this group were initially (Jones, 1901; Yanishevsky, 1927) assigned to the genus Posidonomya Bronn, and later—to the local genus Abiella Rag. At present, among these pelecypods four types of shells can be distinguished, which are easily differentiated from one another when they are not deformed:

a) shells of the nuculoid type with a non-straight hinge margin; in 1939, having established in them a hinge of the parataxodont type, I assigned them to the genus Palaeomutela Amal.; later, taking into account more complete materials on the fossil pelecypods of the Kuzbass, I came to the conclusion that they should be separated into a special genus, which I named Microdonta [5];

b) shells with a long straight hinge margin, small pointed umbones, and flat rounded ears, externally resembling the genus Posidonomya Bronn. I. V. Lebedev [1] established in them the presence of a hinge of the parataxodont type and proposed for them the generic name Microdontella;

c) asymmetrical shells (with an extended lower-posterior margin) with a short straight hinge margin and small ears; “hinge margin... without teeth” [3, 239]; for these specimens the generic name Abiella Rag. may be retained. Typical representatives were figured by D. M. Fedotov [2] on figs. 5, 6, 7, 11 of pl. IX;

d) small, strongly convex shells with broad, inflated umbones, with a tendency toward the formation of ears; apparently, this group is close to the genus Microdontella Leb.; the hinge is unknown; for them we shall use the generic designation Microdontella with a question mark.

These four groups, as already stated, are easily delimited when they are represented by typical and undeformed specimens. But, first, in a number of cases convergent similarity is observed between representatives of different groups, for example, between some microdontas and microdontellas [5], and, second, in the case of deformation (flattening), shells of different genera become similar to one another, and distinguishing them in such a state is not always possible [4, 74]. And since

pelecypods of the Kolchugino fauna are for the most part greatly flattened, then in many cases even generic determinations can be made only approximately; for such determinations we shall use the usual symbol “cf.”, but we shall place it not after the generic name, but before it (for example, cf. Microdontella subovata, etc.).

Microdontella subovata (Jones)

Pl. II, figs. 12—17.

Abiella subovata (in part) Fedotov, 1937, p. 47.
Abiella subovata (in part) Fedotov, 1938, p. 237.
„ concina „ „ 1938, p. 239, pl. VI, figs. 13—16.
Posidonomya subovata Khalfin, 1939, p. 77, pl. VI, figs. 8—16 (see synonymy).
Microdontella problematica Lebedev, 1941, p. 82.
Microdontella subovata Khalfin, 1950, pl. XII, figs. 1—3.

This is the most widespread form of the Kolchugino fauna: it appears already in the upper half of the Tersian horizon of the Kuznetsk suite [5] and passes through the entire Ilyinskaya and Erunakovskaya suites; not infrequently the shells of this species form mass accumulations (Pl. III, fig. 16 and fig. 17). Until now this species was known only from isolated valves or completely flattened shells. From the Baidaevsky deposit the first specimen has been obtained (specimen 63), representing a sculptural internal mold of a complete closed shell. The specimen is somewhat deformed in the direction from the antero-ventral part toward the postero-dorsal angle; because of this the outline of the shell is somewhat distorted (fig. 9 in the text)—the shell is compressed in an oblique dorso-ventral direction. But, fortunately, this deformation has not affected the most important part of the shell—the upper margin with the beaks and auricles. This specimen makes it possible to observe the auricles from the side of the right and left valves and to establish the prosogyrate character of the beaks (Pl. II, fig. 12). A great similarity is revealed in the character of the upper margin in the genus Microdontella and the Balakhonka genus Mrassiella Ragozin emend. [5, Pl. III, fig. 13]. This serves as confirmation of the conclusion I made earlier (cited work) concerning the origin of the Kolchugino microdontellas from the Balakhonka mrassiellas, and makes it possible to anticipate the discovery in the latter of a parataxodont hinge.

Fig. 9. Microdontella subovata (Jones), specimen 63, enlarged; the arrow indicates the direction in which the shell is deformed.

The species Micr. subovata shows great variability in the size of the shell, its outline, and the degree of convexity of the valves. In some beds small representatives of this species predominate, measuring 5—6 mm in length (Pl. III, fig. 16); the usual dimensions are 10—15 mm; considerably larger specimens—up to 20—25 mm in length—are encountered more rarely; these latter are not always found together with the smaller ones and, pending clarification of their relationship to Micr. subovata of ordinary size, deserve to be distinguished as a special form megala (Pl. III, figs. 1—2).

Typical Micr. subovata have rounded, almost isometric outlines, with beaks situated subcentrally on the upper margin. Yet even here the shell is inequilateral: it widens slightly backward, the anterior margin is evenly and markedly convex, while the posterior margin is usually straight or only very weakly convex (Pl. II, figs. 14, 15). But specimens occur with almost equally developed anterior and pos—

... margins (Pl. II, fig. 17), very similar to Micr. cyclos (see below), but in them too the posterior margin in the upper part is straightened, owing to which the auricles are not developed equally (a difference from Micr. cyclos). On the other hand, there are specimens in which the length considerably exceeds the height, so that they are readily distinguished from the typical form; this is the variety elongata (Pl. II, fig. 18). This variety, in the flattened state of the shell, is difficult to distinguish from some representatives of the genus Microdonta (the chief difference in such a state is the subcentral position of the umbones in var. elongata).

With regard to the degree of convexity of the valves, the following may be noted: in typical Micr. subovata the valves are slightly or moderately convex, but there are also specimens with considerably convex valves (Pl. II, fig. 13). It is quite probable that these and the others should be separated taxonomically, but at present there are still insufficient data for such a separation, since the degree of convexity of the valves is the character that, more than all others, is subject to the leveling effect of post-mortem deformation of the shells.

Micr. subovata is the most widespread form of the Kolchugino fauna, occurring everywhere in the areas of development of the deposits of the Ilyinskaya and Yerunakovskaya formations. In the Yerunakovskaya formation of the Baidaevsky deposit it passes through all horizons, being present in almost all faunistic beds; it forms mass accumulations, for example, in the beds: roof of seam 14; 16 m below seam 25; 10 m above seam 26b (several beds); 7 m above seam 26v; 1.35 m above seam 32; 19 m below and 89 m above seam 33; 45 m below seam 34 (several beds); 83 m and 85 m above seam 34. The variety elongata occurs rather rarely, but the form megala accompanies the typical Microdontella subovata in many beds throughout the entire section.

Microdontella cyclos Khalf.

Pl. II, figs. 19–20.

Microdontella cyclos Khalfin, 1950, pl. XVI, fig. 5.

A characteristic feature of this species is the outline of the shell, which is almost a regular circle; by this Micr. cyclos differs from Micr. subovata. This species was established by me in deposits of the Ilyinskaya formation near the village of Kazankova (on the Tom River) and the village of Anisimova (on the Uskat River). It is also present in the Yerunakovskaya formation of the Baidaevsky deposit, where its representatives have larger shells: the Ilyinskaya specimens of this species are 5–7 mm in length, whereas the Yerunakovskaya ones are 10–11 mm. The form is rare; in the Baidaevsky deposit single specimens were found in three faunistic beds: 23 m above seam 25; 48.7 m above seam 32; 62.5 m above seam 33.

Microdontella elliptica Khalf.

Pl. III, fig. 3.

Microdontella elliptica Khalfin, 1950, pl. XVI, fig. 4.

This species differs from Micr. subovata in the development of the shell in length (the outlines are elliptical, not rounded), in the equally rounded (convex) anterior and posterior margins (the chief difference from Micr. subovata var. elongata) and in the weak development of the auricles. Micr. elliptica is often encountered in the Ilyinskaya formation; in the Yerunakovskaya formation of the Baidaevsky deposit it belongs among the comparatively rare forms. Represe...

specimens of this species were found in cores from the following beds: the roof of bed 14 and bed 24, and 19.27 m below bed 33; apparently this species also occurs among the microdontellas in the beds: 10 m above bed 26b and 12 m below bed 26b.

Microdontella (?) protumida n. sp.

Table III, figs. 4—5.

Holotype: 150—Mr, Baidaevsky deposit, Erunakovskaya suite, Baidaevsky horizon, 11 m below bed 27a.

Diagnosis. A small, strongly inflated, isometric shell, rounded in outline, with a straight hinge margin forming small flat auricles. The posterior margin is straightened in its upper part; the ventral and anterior margins are evenly convex. The hinge margin is shorter than the greatest length of the shell; the posterior auricle is subrectangular, more developed than the rounded anterior one. The umbonal part of the valve is inflated, high and broad, occupying the greater part of the upper margin; the beaks are situated subcentrally on the latter. The sculpture consists of broad and flat concentric ridges separated by narrow and sharp grooves; dense and fine concentric lines run along the ridges.

Dimensions of the shells—4—5 mm in length and height.

Comparison. In outline and in the character of the auricles this species is close to Microdontella subovata, but differs in its small size, the greater general convexity of the valves, and especially in their broad and inflated umbonal region. Externally it is very close to Microdonta plotnikowskіensis Fed., but apparently this resemblance is convergent: in Micr. plotnikowskiensis the hinge margin is not straight and there is a hinge characteristic of the genus Microdonta. In the species described here the auricles are quite distinctly developed, and the hinge margin is straight—features characteristic of the genus Microdontella. But since the hinge of this species remains unstudied, its generic assignment cannot be precisely determined.

Distribution of this form is confined to a limited stratigraphic interval within beds 26—28, but Micr. (?) protumida is usually characterized by mass development within a given bed, namely—in the beds: 6.8 m above bed 26b; about 14 m above bed 28 (several beds).

Microdonta baydajevskiensis n. sp.

Table III, figs. 6—10.

Holotype: 90—Mb, Baidaevsky deposit, Erunakovskaya suite, Abashevsky horizon, 2 m above bed 25.

Diagnosis. A shell of medium size, moderately convex, elongated in length (its height is about 3/4 of the length), subtrapezoidal in outline. The shell widens slightly posteriorly owing to the fact that its posteroventral end, although broadly rounded, is slightly lowered, while the ventral margin is weakly convex, in some specimens even straight in the middle part. The posterior margin is broadly rounded; the anterior one is narrower. The beaks are moderately high, considerably approximated to the anterior margin (the distance between the beaks and the anterior margin is 1/4—1/3 of the shell length). The valves are evenly convex, without a distinct keel. The surface of the valves is covered with low concentric ridges, which are separated by narrow grooves and along which pass fine, dense concentric lines.

Measurements:	Shell length	Shell height	Distance from umbones to anterior margin	Specimen no.
Measurements:	13	9	3	90 (holotype)
Measurements:	16.5	12	5	90 (holotype)

Comparisons. Microdonta microdonta Khalf. [4,72], widespread in the Il’inskaya Formation and in the lower part of the Erunakovskaya Formation, differs in having umbones more distant from the anterior margin and a strongly convex ventral margin, owing to which the outlines of the shell in this species are rounded-triangular.

Remarks. Flattened shells of this species are difficult to distinguish from some microdontids having elongated valve outlines, for example Micr. subovata var. elongata. Micr. baydajeveskiensis differs from them above all in having umbones more shifted toward the anterior margin (pl. III, fig. 7).

As a rule, this form occurs in the deposits of the Baidaevsky locality as isolated valves; an exception is specimen 407 (pl. III, fig. 7), which is an opened shell with the valves not separated: the latter touch along the hinge margin behind the umbones, indicating the position of the ligament in the genus Microdonta.

Distribution. Micr. baydajeveskiensis passes through all three horizons of the Baidaevsky locality (beginning with the Abashevsky), but it is represented most typically and abundantly in the roof of seam 25. The first specimens, which probably (being strongly flattened) belong to this species, appear between seams 12 and 13 (pl. III, fig. 7), and also in the roof of seam 14. Subsequently this species has been established approximately 1.5 m above seam 20. Above seam 25 Micr. baydajeveskiensis was found in the following beds: 1.75 m, 1.95 m, and 2.25 m above the seam. Further, representatives of this species occur in beds above seam 26a, 14 m above seam 28, and 15 m below seam 33. In the very extensive collections available to me from various areas of development of the Il’inskaya Formation, this species has not been found, and therefore it should be assigned to the number of guide forms for the Erunakovskaya Formation.

Microdonta longa n. sp.

Pl. III, figs. 11—12.

Holotype: 184—M1, Baidaevsky locality, Erunakovskaya Formation, Esaulsky horizon, 50 m above seam 33.

Diagnosis. Shell long and low, very inequilateral: anterior end short, posterior very long. The greatest height of the shell, passing through the umbo, is less than half the length. Posteriorly the shell gradually narrows, but the posterior margin is not pointed, but rounded, like the anterior. The ventral margin is evenly and strongly convex throughout its entire length. Low, blunt umbones are distant from the anterior margin by a distance equal to 1/4 of the shell length. The convexity of the valves is moderate and regular. The outer surface is covered with irregular concentric ridges and fine concentric lines.

Measurements:	Shell length	Shell height	Distance from umbo to anterior margin
Measurements:	15	7	4	(holotype)

Comparisons. This species is easily distinguished from Micr. baydajevskiensis and Micr. microdonta, above all by its strongly elongated and very thin shell.

Distribution. This rare form has been found only in deposits bordering between the Baidaevsky and Esaulsky horizons, namely in beds 10.3 m above seam 32 and approximately 50 m above seam 33.

CONCLUSION

The paper describes a collection of lamellibranch mollusks gathered in the deposits of the Erunakovskaya Formation of the Baidaevsky deposit in the Kuzbass. The collection consists of 187 core samples, covering the stratigraphic interval from seam 12 through seam 38 inclusive and containing thousands of specimens of pelecypods. Thus, according to the scheme proposed by E. M. Senderzon, the fauna studied embraces the three upper horizons of the Erunakovskaya Formation—the Abashevsky, Baidaevsky, and Esaulsky—and only the very lowest horizon (the Feskovsky) is not characterized by fauna in the collection studied.

The general list of forms described in the paper is given on p. 107; it consists of 24 names, of which 13 belong to new species and varieties. The most important group of Erunakovskaya anthraconauts—the group Anthr. pseudophillipsii Fed.—has been subjected to special consideration; for its representatives a scheme of genetic relationships is presented (Fig. 5 in the text, p. 114), and the origin of certain specific features as the result of uneven growth of the shell in different directions is clarified (Fig. 8 in the text, p. 115).

According to the literature data, only 9 species of pelecypods had so far been known from the Erunakovskaya Formation; of these, only two can be considered characteristic solely of this formation. The study of the Baidaevsky collection has not only considerably increased the number of forms distributed in the Erunakovskaya Formation, but has also made it possible to establish guiding complexes of pelecypods characterizing both the Ilyinskaya and the Erunakovskaya formations (each separately); the corresponding lists are given on p. 109. Further, by tracing the stratigraphic distribution of the species described in the paper, it appears possible to establish a number of forms characterizing narrow stratigraphic intervals; this is shown graphically in Fig. 1 (p. 110), and the corresponding materials are given on p. 110. This is the first attempt at a zoostratigraphic subdivision of the Erunakovskaya Formation; the data obtained on this question must be verified by studying bed-by-bed collections of fauna in some other section of this formation.

Facies analysis of the fauna and of the state of its preservation permits the conclusion that the deposits containing this fauna were formed in freshwater basins of the lake type, and that only an insignificant part of them was deposited in flowing waters. In this respect they differ noticeably from the rocks of the Ilyinskaya Formation of the same region, which were formed under conditions of great instability and fluctuations in the character of the corresponding basins.

References Cited

1. Lebedev I. V. On some lamellibranchs from the Kolchugino series of the Kuznetsk Basin, Izv. AN SSSR, biological series, No. 2, 1944.

2. Fedotov D. M. Lamellibranch mollusks of the Carboniferous deposits of the Kuznetsk Basin, Transactions of the Central Scientific-Research Geological Prospecting Institute, issue 97, 1937.

3. Fedotov D. M. Pelecypods from the Carboniferous deposits of the industrial regions of the Kuznetsk Basin, Izv. AN SSSR, biological series, No. 1, 1938.

4. Khalfin L. L. Materials for the study of pelecypods of the Kolchugino series of the Kuznetsk Coal Basin, Izv. Tomsk Industrial Institute named after S. M. Kirov, vol. 60, 1939.

5. Khalfin L. L. Fossil pelecypods of the Kuznetsk Basin, 1950.

6. Jones T. R. On some Carboniferous Shales from Siberia, Geol. Mag., N. S., dec. IX, vol. VIII, 1901.

EXPLANATION OF PLATES

All figures, with the exception of those specially noted in the explanations, are given at natural size. All photographs presented on the plates belong to specimens from the deposits of the Erunakovskaya Formation of the Baidaevsky locality, except for figs. 3, 13, 17, 18 on Plate III.

Plate I

Figs. 1—8. Anthraconauta pseudophillipsii Fed. emend. Figs. 1—3—typical specimens: fig. 1—spec. 390, roof of seam 14; fig. 2—spec. 132, roof of seam 25; fig. 3—spec. 75, 37.55 m above seam 33; fig. 4—spec. 405-1, transitional to the variety angusta; roof of seam 25. Figs. 5, 6—specimens transitional to the species Anthr. ampla: fig. 5—spec. 48, 45 m below seam 34; fig. 6—spec. 149, 12.55 m above seam 26b. Figs. 7, 8—specimens transitional to the variety acuta: fig. 7—spec. 18-1, 12 m below seam 26b; fig. 8—spec. 18-2, from the same place.

Figs. 9—11. Anthraconauta pseudophillipsii Fed. n. var. angusta: fig. 9—spec. 405-2, roof of seam 25; fig. 10—spec. 417, roof of seam 25; fig. 11—spec. 156, holotype, 8.4 m below seam 23.

Figs. 12—13. Anthraconauta pseudophillipsii Fed. var. acuta Khalf.: fig. 13—spec. 63, typical specimen, 6.8 m above seam 26b; fig. 12—spec. 18-3, 12 m below seam 26b, enlarged.

Figs. 14—16. Anthraconauta pseudophillipsii Fed. var. subaequalis Khalf. Fig. 14—spec. 43, 55 m above seam 33; fig. 15—spec. 45-1, 55 m above seam 33; fig. 16—spec. 140, very large specimen, 6.7 m below seam 16.

Fig. 17. Anthraconauta cuneata n. sp., holotype; 25 m above seam 22.

Figs. 18, 19. Anthraconauta simplex Khalf.: fig. 18—spec. 159, 81.5 m above seam 32; fig. 19—spec. 120, 19.26 m below seam 33.

Fig. 20. Anthraconauta lata Khalf., spec. 155, 43.2 m above seam 33.

Figs. 21—22. Anthraconauta ampla n. sp. fig. 21—holotype, 29.5 m above seam 26b; fig. 21a—holotype, enlarged; fig. 22—spec. 133, 18 m above seam 25.

Plate II

Figs. 1—3. Anthraconauta propinqua n. sp.: fig. 1—holotype, 16 m above seam 26b; fig. 1a—holotype, enlarged; fig. 2—spec. 21, 15 m below seam 26a; fig. 3—spec. 27, young specimen, enlarged, 7 m below seam 26b.

Fig. 4. Anthraconauta inflata n. sp. holotype, 21.4 m above seam 32, spec. 199/32.

Fig. 5. Anthraconauta kostomanovi n. sp., holotype, 19.25 m below seam 33, spec. 119/33.

Fig. 6. Anthraconauta subtenuis n. sp., holotype, spec. 422/12, roof of seam 12.

Fig. 7. Anthraconauta subparallela Khalf. n. f. postera, holotype, enlarged; 45 m below seam 34, spec. 127/34.

Figs. 8—9. Anthraconauta tchernychevi Khalf.: fig. 8—spec. 104, 18.65 m above seam 26b; fig. 9—spec. 46, 15 m below seam 33.

Fig. 10—11. Anthraconauta n. sp. cf. A. tchernychevi Khalf.: fig. 10—specimen 73, 10 m above bed 26b; fig. 11—holotype, 19.5 m below bed 33.

Fig. 12—17. Microdontella subovata (Jones): fig. 12—specimen 69, a somewhat deformed sculptured internal mold of a shell in three projections, enlarged, 2.35 m above bed 25; fig. 13—specimen 418, a well-preserved sculptured internal mold of the left valve, between beds 22 and 23; fig. 13a—the same specimen, enlarged; fig. 14—specimen 407, between beds 12 and 13; fig. 15—specimen 190, 41.8 m above bed 29a; fig. 16—specimen 11, 10 m below bed 25; fig. 17—specimen 394, a strongly rounded shell resembling Microdontella cyclos; between beds 13 and 14.

Fig. 18. Microdontella subovata (Jones) var. elongata Khalf., specimen 53, 0.05 m above bed 33.

Fig. 19—20. Microdontella cyclos Khalf.: fig. 19—specimen 4, 23 m above bed 25; fig. 20—specimen 202, enlarged, 48.7 m above bed 32.

Plate III

Fig. 1—2. Microdontella subovata (Jones) forma megala Khalf.: fig. 1—specimen 40, 26.65 m above bed 34; fig. 2—specimen 388, roof of bed 14.

Fig. 3. Microdontella elliptica Khalf.: specimen 270, enlarged, Ilyinskaya suite of the Abashevo district.

Fig. 4—5. Microdontella (?) protumida n. sp.: fig. 4—holotype, 11 m below bed 27a; fig. 4a—holotype, enlarged; fig. 5—specimen 20, 14 m above bed 28; fig. 5a—the same specimen, enlarged.

Fig. 6—10. Microdonta baydajevskiensis n. sp.: fig. 6—holotype, 1.95 m above bed 25; fig. 7—opened shell, specimen 407, between beds 12 and 13; fig. 8—specimen 79, 1.5 m above bed 20, enlarged; fig. 9—specimen 74, 2.25 m above bed 25; fig. 10—specimen 99, 15 m below bed 33.

Fig. 11—12. Microdonta longa n. sp.: fig. 11—holotype, 50 m above bed 33; fig. 12—specimen 111, 10.3 m above bed 32.

Fig. 13. Abiella concinna (Jones), Ilyinskaya suite of the Abashevo district, enlarged.

Fig. 14. Mass accumulation of Anthraconauta pseudophillipsii Fed., 30.9 m above bed 33.

Fig. 15. Shells of Anthraconauta pseudophillipsii Fed., arranged in a chain along the direction of the water current; 28 m above bed 33.

Fig. 16. Mass accumulation of small shells of Microdontella subovata (Jones); 45 m below bed 34, specimen 109/34.

Fig. 17. Mass accumulation of large shells of Microdontella subovata (Jones); Ilyinskaya suite of the Abashevo district.

Fig. 18. Fragments of shells and leaves of cordaites, oriented along the direction of the water current; Ilyinskaya suite of the Abashevo district.

Plate I

1 2 3 4

5 6 9 10

7 8 11 17

12 13 14 15

18 19 20 16

21

Table 11

Plate III

Visible figure labels: 1, 2, 3, 4, 5, 5a, 6, 7, 8, 9, 10, 15, 11, 12, 13, 14, 16, [[unclear: lower-left figure label]], [[unclear: lower-right figure label]].

1. A form which, though rare, is clearly connected with the two preceding species. ↩