V. A. Khakhlov, mining engineer.

The Jurassic Group Ginkgoales of the Irkutsk Coal-Bearing Basin

PREFACE

Plant remains of the Irkutsk Basin were collected in fairly considerable quantity by parties of the Siberian Branch of the Geological Committee in 1919, 1920, and 1921, the general direction of which had been entrusted to the geologist M. K. Korovin. In 1919 the paleophytological material was collected chiefly in the eastern part of the basin, namely in the region of the Angara River. In subsequent years the work of the parties was concentrated in the industrial district of the basin itself. It should be noted that the district of the mines is characterized by a poverty of plant material, in comparison with the non-industrial part of the basin, where along the banks of the Angara River and its tributaries there are quite a considerable number of outcrops of the Jurassic series with numerous impressions of plants. Part of this material, namely the group of Jurassic ferns, has already been treated by the trainee A. I. Turutanova, and the results published in the Proceedings of the Siberian Geological Committee, Vol. I, issue 5. During 1922 I carried out further treatment of the plant remains, and the present work is devoted to representatives of the group Ginkgoales from the class Ginkgoinae.

The group Ginkgoales in the Irkutsk Jurassic is represented by the following forms: Ginkgo Huttoni Sternb., Ginkgo Digitata Heer, Ginkgo Sibirica Heer, Ginkgo sp., Baiera longifolia Pom., Baiera Czekanowskiana Heer, Baiera Gracilis Bunb., Baiera pulchella Heer, Baiera sp., Czekanowskia rigida Heer.

The prevailing importance in the collection is held by representatives of the genus Czekanowskia Heer, which accounts for about 400 specimens. The genus Baiera F. Br. is represented by comparatively few impressions; and, finally, representatives of the genus Ginkgo L. occur in very scanty quantity.

Before proceeding to the description of the impressions of Jurassic plants, I must say that I have taken into consideration the remarks of the Cambridge University professor A. C. Seward concerning certain species of the Middle Jurassic flora collected by F. B. Schmidt and P. Glebov in the upper course of the Amur River and on the Bureya River and described by Professor O. Heer: he expresses the wish that Heer’s species should be revised, which, in his opinion, ought to lead to a reduction in their number. Further, in treating the Jurassic plants of the Caucasus and Turkestan, he proposes to understand the species of the genus Ginkgo L. in a broader sense. On this subject he says: “The description, under different specific names, of various forms of leaves more or less closely resembling the surviving member of the group Ginkgoales, although necessary for a certain evaluation, nevertheless presents a striking example of the artificiality of the methods that must be used owing to the fragmentary and insufficient nature of the material with which we have to deal. Examination of a series of leaves from different shoots or at different seasons from the trunk of Ginkgobiloba L. indicates the futility of basing specific characters on the degree of lobation or on the size of the leaf blade” (pp. 14–32). Fully agreeing with the opinion of Prof. A. C. Seward, and taking into account the fact of the existence of individual variability in almost all organisms of the animal and plant kingdoms, I regard specific differences and understand them in a broader sense within the limits not of a single genus...

Ginkgo L., while the whole group Ginkgoales, as a group very readily reacting to external environmental influences and, apparently, characterized by stronger variability than its surviving modern primitive representative.

Description of plant remains.

Genus Ginkgo L.

The genus Ginkgo L. first occurs in Permian deposits, although it is possible that some of its representatives began their existence in the upper division of the Carboniferous system. It reached luxuriant development in the Jurassic period; in the Cretaceous it is represented by few forms. At the present time there occurs only a single representative, Ginkgo biloba L., distributed chiefly in Japan and China, with simple, very weakly dissected leaves. The extinct representatives are characterized, in the main, by more deeply dissected leaves and by great diversity in their external form, so that it is often difficult and even almost impossible to draw a boundary between individual representatives of this genus.

The leaves of the genus Ginkgo L. consisted of a broad lamina, divided by a deep sinus into two, and sometimes into three, parts, each of which in turn again was divided by a deep incision into 2 lobes. Such dissection of the leaf lamina may reach three times, with the outer lobes being the most inclined to division, so that while they are only beginning to divide, the inner lobes remain entire, and only when the dissection of the outer lobes attains considerable size do the inner lobes begin to acquire a corresponding incision at the end of the lobe. In Fig. 7 we observe just such a case, when the leaf is first divided by a very deep sinus into 2 parts, of which the right part is further divided by a rather deep incision into two lobes or segments, so that the leaf lamina appears to consist of four separate leaflets, of which the outer ones have a tendency toward further dissection, while the inner ones remain entire-margined. The leaf segments produced as a result of division of the original leaf lamina always have an almost elongate-oval form.

The broad leaf lamina, gradually narrowing toward the base, passes into a thin leaf petiole of rather considerable length. Into the leaf there enters fanwise a small number of rather coarse veins, which at once begin to divide dichotomously. Their secondary forked branching occurs when the leaf is divided into separate segments. In some cases the veins divide once again in the middle of the leaf lobes, after which they run parallel and only at the end of the lobe begin again to converge, following its outer outline. It is understandable that the number of veins after threefold dichotomous division becomes considerable, often reaching twenty or more in broad-lobed and weakly dissected forms. Usually this number in the broad part of a leaf segment reaches twenty and is often equal to six to eight, and comparatively rarely 2–3. The distance between the veins is insignificant and only in the rarest cases equals 2 millimeters. More often the veins are situated at a distance of one millimeter from one another. Between the veins in the modern and in some fossil representatives, for example in Ginkgo sibirica Heer, there are transverse wavy wrinkles. Also observed here and there are longitudinal interrupted grooves running parallel to the vascular bundles (15, p. 16). The transverse stripes approaching the veins at right angles represent de-

organized and contracted mesophyll situated between the lower and upper cuticles. The longitudinal stripes correspond to secretory canals.

The leaves were arranged on short branches together with stipules and flowers.

The male flowers were in the form of catkins reaching thirty millimeters in length. The stamens are directed somewhat forward and bear at the end heads consisting of 2–3 anthers. Male flowers in large numbers were found in Eastern Siberia and the Amur region and were described in greater detail by Professor O. Heer. Their entire length, together with the stalk, is usually 46 millimeters. The stalk accounts for 15 millimeters and the part bearing the anthers for 25–30 millimeters. The stalk is thin, rather coarse, and striate.

The genus Ginkgo L. in the Mesozoic era apparently had a large number of distinct forms, and their separation is based on various features and degrees of dissection of the leaf.

Ginkgo Huttoni (Sternb.) Heer.

Figs. 1, 2.

1. Ginkgo Huttoni (St.) Heer. F. Krasser. (27), p. 12. Pl. IV, figs. 3 and 4.

2. Ginkgo Huttoni (St.) Heer. O. Heer. (24), p. 59. Pl. VII, fig. 4; Pl. X, fig. 8.

The specimen shown in fig. 1 represents only the left half of the leaf blade, the right half having been broken off. Nevertheless it can still be observed that by a deep sinus the leaf is divided into two halves, each of which is dissected once again by a less deep incision; thus the leaf apparently consisted in all of four lobes. The outer lobe is 35 mm long and reaches 14 mm in width at its widest point. At its end it has a small notch 7 mm deep. In its widest part 10 longitudinal veins are counted. The inner lobe is 12 mm wide, and through it run 9 rather distinct veins. The distance between the veins is almost 1.5 mm. Below, a fragment of the leaf stalk is visible, 1.5 mm wide. Three veins enter each lobe from the stalk, and these immediately branch dichotomously. Secondary branching occurs in the lower third of the leaf, after which the veins run parallel and only in the very upper part of the lobe begin to converge rapidly, following its outer outline.

The specimen agrees fairly well with F. Krasser’s description. It differs from Ginkgo digitata Hr. in its shape, in the wider spacing between the veins, and in their small number.

Locality: right bank of the Angara River, 1.5 versts above the island of Konlanovsky, 3–4 versts from Sukhovo station of the Transbaikal Railway.

Figure 2 depicts a fragment of a leaf. The width of the lobe reaches 15 mm. The veins are arranged at a distance of 1.5 mm. Their number in the left lobe is 10–12; in the right part it is possible only to determine the distance between them, likewise 1.5 mm, and to observe their dichotomous branching. Owing to the poor preservation of the specimen, their number cannot be counted. The veins are very thin but distinct.

Locality: argillite bed in the northwestern wall of the open cut of the Nagalinsky mine.

Ginkgo digitata Heer.

Fig. 3, 4.

1. Ginkgo digitata Hr. A. Seward. (¹⁵). P. 17. Pl. III, fig. 40.
2. Ginkgo digitata Hr. H. Hamshaw Thomas. (¹⁸). P. 26. Pl. IV, fig. 7; Pl. VIII, fig. 2.
3. Ginkgo digitata Hr. A. Seward. (¹⁴) P. 32. Pl. VII, figs. 53—55.

The leaf blade (fig. 3) is divided by a narrow and comparatively shallow sinus into two broad lobes, the apices of which are broken off, but traces are nevertheless visible of their dissection into two parts by an even shallower notch. As a result of the twofold dissection the leaf appears to consist of four separate lobes, on which numerous, fine, but fairly sharp veins are very clearly visible. Their number on the inner lobes reaches 20. The distance between them is 1 mm. Dichotomous branching is also noticeable in the middle of the leaf. The specimen agrees fully with the illustration in H. Hamshaw Thomas, Pl. VIII, fig. 2.

Locality: right bank of the Angara River, between Strekalovskii Island and Shutkina winter settlement.

Another specimen (fig. 4) represents the right half of a leaf blade, broken off at both ends. Nevertheless, it is still evident that it consists of two lobes, of which the right one is again divided at the end by a small sinus, 9 mm deep, into two parts, of which the outer one has a blunt-oval outline. The greatest width of the right lobe is 18 mm, and through it pass, at a distance of 1 mm from one another, 18 distinct, fine, parallel veins. Dichotomous branching is also present in the middle of the leaf. The leaves of these specimens are less dissected than the leaves which I described under the name Ginkgo Huttoni Hr., and have more numerous veins. They are almost identical with O. Heer’s illustration in Pl. X, fig. 3a (24), and their only slight difference is the presence of a sinus on the outer lobe of the leaf, whereas in Heer’s specimen it appears entire.

Locality: right bank of the Angara River between Shutkina winter settlement and Kompaneiskii Island.

Ginkgo sibirica Heer.

Fig. 5—8.

1. Ginkgo sibirica Hr. Heer (²⁴). P. 61. Pl. VII, fig. 6, Pl. IX, fig. 5b; Pl. XI.
2. Ginkgo sibirica Heer. A. Krishtofovich. (⁶). Vol. II. Issue IV, p. 116.
3. Ginkgo sibirica Hr. A. Seward. (¹⁶). P. 5.
4. Ginkgo sibirica Hr. A. Seward. (¹⁶). P. 33. Pl. VII, figs. 56, 57.

Leaves of the genus Ginkgo L., collected in Irkutsk Province and the Amur Region and referred by O. Heer to Ginkgo sibirica Hr., Ginkgo lepida Hr., Ginkgo flabellata Hr., Ginkgo pusilla Hr., and Ginkgo Schmidtiana Hr., do not have well-expressed characters on the basis of which they could be divided into separate species, and Prof. A. Seward considers that a revision of these species should lead to a reduction in their number. A. Seward himself repeatedly described some leaves of the genus Ginkgo L. under

under the name Ginkgo sibirica Hr., and consequently, of all these names, Ginkgo sibirica Hr. is, as it were, retained; thus it represents a rather extensive species with considerable modifications of the leaf blade. In addition, the species Ginkgo sibirica Hr. is widely distributed in Jurassic deposits throughout the world and links the forms included in the genera Ginkgo and Baiera F. Br. It has been found in Siberia, Turkestan, Franz Josef Land, Manchuria, Japan, North America, and Australia.

Ginkgo sibirica Hr. differs from Ginkgo Huttoni (St.) Hr. and Ginkgo digitata Hr. in having a more strongly dissected leaf blade and narrower leaf lobes with few veins and bluntly terminating ends.

In Fig. 5 the leaf blade is first cut into two deep lobes, each of which in turn is again divided into two parts by a deep incision, almost reaching the base of the stalk, so that it seems to us that the leaf consisted, as it were, of four separate leaflets. The leaf lobes are elongate-oval in form, gradually narrow toward the base of the leaf blade, and end at the apex bluntly oval. On the outer lobes the presence of a sinus 2–3 mm deep is noticeable. The inner lobes remain entire-margined. The length of the lateral lobes is 25 mm, with the greatest width 7 mm. The length of the inner ones is 24 mm, with the greatest width 5 mm. The veins are thin and rather distinct. Their number on the outer lobes is 8, and on the inner ones 6. Between the veins transverse wrinkling is noticeable.

In Fig. 6 an incomplete leaf blade is presented, apparently cut into four separate lobes. Of these, only three have been preserved. The lobes have an oval-elongate form and end bluntly. The inner ones are longer than the outer ones and reach 22 mm, with the greatest width 5 mm, whereas the outer ones, with the same width, are 18 mm long. The specimen is rather poorly preserved. The veins are only weakly noticeable; nevertheless, in one middle lobe, in its widest part, they can be counted. Their number reaches 6–7.

Fig. 7 depicts a leaf which to some extent resembles leaves of Ginkgo digitata Hr., if one adheres only to A. C. Seward’s view and understands species in a broader sense, i.e., considers that leaves of Ginkgo digitata Hr. may have narrower lobes and be more dissected than those forms described by various authors under the name Ginkgo digitata Hr. In any case, here we have a leaf form, transitional, which with equal probability may be assigned both to Ginkgo digitata Hr. and to Ginkgo sibirica Hr. But, adhering to the descriptions of most authors, including the description by O. Heer himself, I nevertheless assign this form to Ginkgo sibirica Hr. on the basis of the great dissection of the leaf blade, as a result of which the individual lobes have acquired a narrower form with a smaller number of parallel veins. The rather broad leaf blade (Fig. 7) is divided by a deep incision into two parts—the right and the left. Each of the latter, in turn, is cut into two unequal parts: the inner ones are narrower, reaching 6 mm in the widest part, and the outer ones 8–9 mm wide. The outer lobes show a tendency to divide once again. The length of the outer lobes is 30 mm, that of the inner ones 22–24 mm. The veins are distinct and thin. Their number on the outer lobes is 8, on the inner ones 6. The distance between them is 1 mm.

An even greater dissection of the leaf blade is presented in Fig. 8, where the leaf appears divided into 6 separate lobes. Such a form of the leaf blade is easy to derive from the preceding one, shown in Fig. 7, if one imagines that its outer leaves are cut by a very deep sinus, so that the leaf lobes have already acquired a certain independence. In the broad part of the leaf blade there pass 6–7 longitudinal parallel veins with distinct dichotomous branching.

Locality: Right bank of the Angara River, above Konshaneisky Island, and the open section of the Grishevo mines.

Ginkgo sp.

Fig. 9.

The specimen is rather poorly preserved. Only two lobes are visible, each of which is cut at the ends by a shallow sinus. The venation is poorly visible. The number of veins cannot be counted, but nevertheless, from individual fragments one may judge that they are not numerous. The distance between them is 0.5 mm. At the base of the leaf blade their forked division is observed. The length of the lobes is 20 mm, with the greatest width 8 mm. The lobes gradually narrow toward the base of the leaf. It must be said that the preserved part of the leaf blade has a rather original form and has no counterpart in descriptions of the Jurassic flora of various regions of the globe. The incomplete leaf blade is represented by only a single specimen, and one of poor preservation at that; therefore I consider it premature to distinguish it as a special new form.

Locality: Right bank of the Angara River between Strekalovsky Island and Shchutin’s zaimka.

Genus Baiera F. Br.

Representatives of the genus Baiera F. Br. are widely distributed in almost all parts of the globe. They appeared as early as the Permian period, reached luxuriant development in the Upper Triassic and Jurassic, and from the end of the latter their rapid extinction is observed. By the end of the Cretaceous period typical representatives of them no longer existed.

The genus Baiera F. Br. differs from Ginkgo L. in having a shorter leaf stalk, a narrower leaf blade gradually merging with the petiole. The leaves are more deeply divided and have narrower leaf segments, in which parallel veins run in varying number. The leaf lobes, almost with parallel margins, end bluntly or sometimes sharply. The comparatively narrow leaf blade may be divided into only two narrow parts, each of which remains entire. In some cases each of these segments, in turn, divides dichotomously into two parts, so that a leaf divided into only 4 “segments” is obtained. In comparatively more frequent cases the outer segments of these four divide further, whereas the inner ones remain simple. Quite often the leaf blade is very strongly dissected, and the number of segments composing it reaches 30 and more. It should be noted that the variability of form and the unequal degree of its dissection are a rather common phenomenon not only for representatives of the genus, but also for individual specific units. Therefore the outline and degree of dissection are not characters on the basis of which species could be well distinguished. In the leaf segments run comparatively numerous, parallel, rather distinct veins, which branch dichotomously only once at the base of the leaf. Between these veins, continuously and in rows, under strong magnification, intermediate parallel, thinner veinlets are observed.

The leaves were situated at the ends of short branches, as is also the case in extinct and modern representatives of the genus Ginkgo L. At their bases were stipules and clusters which to some extent recall the catkins of the genus Ginkgo L.

Baiera longifolia Pom. sp.

Figs. 10–15.

1. Baiera longifolia Pom. O. Heer. P. 52. Pl. VII, figs. 2, 3; Pl. VIII; Pl. IX, figs. 1–11; Pl. X, figs. 6, 7; Pl. XV, fig. 11-b.

2. Baiera longifolia Pom. sp. A. Krishtafovich, (7). P. 110, pl. VI, figs. 1, 2.

I assign to this species a rather large number of specimens. Fig. 10 shows an incomplete leaf, the right part of which and the ends are broken off. The specimen is quite similar to the specimen figured by Heer on pl. VII, fig. 2. It is evident that the leaf was at first divided into two comparatively narrow parts, which reach a width of 12 mm, and then again divide into two parts, forming leaf lobes with parallel sides, 7–8 mm wide. The number of longitudinal parallel veins in the narrower lobes is 7–8; in the broad ones it is considerably greater and is usually 12.

Fig. 11 depicts a leaf dissected into two parts, which in turn again divide dichotomously into two parts, forming lobes: the outer ones 12 mm wide, and the inner ones 8 mm wide. About 10 veins pass through the latter; in the outer, broader ones, 12–14 veins. It is possible that this number is not entirely exact, since the specimen is rather poorly preserved and the venation is considerably worn away.

Figs. 12, 13, and 14 depict the ends of leaf lobes.

In Fig. 12 the leaf segment is divided into unequal parts: the outer ones reach 6 mm in width, while the inner ones are only 4 mm wide. The specimen is poorly preserved. The number of veins can be counted only in the outer lobes, where it is 6–7.

In Fig. 13 a broad leaf blade is represented incompletely. It seems to be divided in all into two parts, of which the left has been preserved completely, while the right is broken off. Possibly this is only part of a leaf lobe, and not the entire blade. The end of the preserved lobe is bluntly oval-rounded in shape, 7 mm wide, and seven rather distinct parallel veins pass through it.

The specimen represented in Fig. 14 differs in its narrower dimensions and in the greater number of closely spaced veins. The width at the base of the lobe is 4 mm; before the dichotomous branching it widens somewhat and reaches 6 mm. After division the lobes are 2–2.5 mm wide, bluntly oval-rounded, and an unequal number of parallel veins pass through them: in the right lobe there are 5, in the left—4. In the main lobe their number is 8.

The specimen shown in Fig. 15 is distinguished by a somewhat peculiar form of the leaf blade. It recalls fig. 6, pl. VIII of O. Heer. A narrow leaf blade is divided by a very narrow sinus into two parts. Before division its width is 10 mm. After division the width of the lobes is 5 mm, and 6 veins pass through each of them. Between the longitudinal veins, finer intermediate parallel veinlets are visible under a lens; in places these are crossed by transverse grooves, producing a wave-like wrinkling.

Locality: The main number of specimens was found along the right bank of the Angara River near Strekalovsky Island. Several specimens come from the Jurassic deposits exposed at the mouth of the Irkut River, and several specimens were found in the open quarry of the Andreevsky mine.

Baiera Czekanowskiana Hr.

Figs. 16–22.

1. Baiera Czekanowskiana Hr. O. Heer.(24) p. 56. Pl. X, figs. 1–5; pl. VII, fig. 1.
2. Baiera Czekanowskiana. Hr. Heer (23) p. 12. Pl. 1, fig. 12; pl. 11, fig. 1; pl. III, figs. 4–8.

The specimens that I assign to this species differ from the preceding ones in having narrower and more sharply pointed leaf lobes, through which passes an insignificant number of parallel veins, equal to 4. The best specimen is shown in fig. 16. The leaf blade divides, deep down, into two parts. Then each of them again dichotomously divides into two parts, and this division of them reaches three times in each half of the leaf. The outer lobes have a width equal to 4 mm. In some places they broaden and attain a width of 5 mm; in other cases, on the contrary, they become narrower, barely reaching 3 mm. The ends of the lobes are broken off, and therefore nothing can be said about their shape. The number of veins is 4. Under strong magnification, intermediate, finer parallel veins are observed.

Other specimens fully supplement the structure of the entire leaf blade. Thus, in fig. 17 we have the completely preserved endings of the leaf lobes. They gradually narrow toward the apex and end more sharply. The width of the lobes varies from 2.5 mm to 4 mm, while before the dichotomous branching of the lobes their width increases very noticeably and reaches 7 mm. Figs. 18, 19, 20, 21, and 22 also belong to this species. On some specimens the width of the lobes reaches 5–6 mm, but the number of longitudinal veins remains constant and equal to 4.

Many of the specimens cited differ somewhat from Heer’s by their broader leaf lobes. They to some extent transgress the limits established by O. Heer, who says that the width of the lobes of the leaf blade of Baiera Czekanowskiana Hr. is not more than 4 mm and that they always have up to 4 longitudinal parallel veins. Taking into account the rather considerable variability in the width of the leaf lobe even within the limits of a single leaf blade, it seems to me that it will be quite proper to include specimens with broader lobes in this species as well, thereby extending its limits with respect to the width of the individual lobes and, consequently, understanding it in a broader sense.

Locality: Andreevsky mine; roof of the main seam.

Baiera pulchella Hr.

Figs. 23–26.

1. Baiera pulchella Hr. O. Heer. (24). p. 114. Pl. XX, fig. 3; pl. XXII, fig. 1-a; pl. XXVIII, fig. 3.
2. Baiera pulchella Hr. O. Heer. (23). p. 115. Pl. IV, figs. 3, 4.

I have at my disposal several specimens which differ in the width of the leaf lobes from Baiera longifolia Pom. sp. and come very close to Baiera pulchella Hr. The very narrow leaf blade is dissected into only two lobes, which remain entire and do not branch further. The width of the lobes after division is 9 mm, and in it there are from 12 to 14 longitudinal veins. Then the leaf lobes begin noticeably to broaden and in the middle of the lobe reach 14 mm, after which they again begin gradually to narrow. In the broadest part there are 14–16 longitudinal, sharper, parallel veins. They attain particular sharpness in the spec—

lobe shown in Fig. 25, where the width in the middle of the lobe reaches its maximum value. It is 18 mm, and 18 veins pass through it, whereas at the base of the division of the leaf blade its width is only 12 mm. The ends of the lobes are either bluntly oval or more or less pointed. The leaf blade gradually passes into the petiole.

Locality: Right bank of the Angara between Strelalovskii Island and Shutkina zaimka.

Baiera gracilis Bunb.

Figs. 27, 28.

1. Ginkgo sibirica Hr. A. Krishtofovich (⁶). P. 116, Pl. X, fig. 7; pl. II, fig. I.

2. Baiera gracilis Bunb. Le Marquis de Saporta. (³⁰). P. 277. Pl. 157, fig. 4; pl. 158, figs. 1—3.

In Fig. 27 is shown the best specimen, reaching 11 cm in length. The leaf blade, by a very deep sinus, is divided into two parts, which are again divided by an equally deep incision into two unequal parts: the outer ones broader, the inner ones narrower. In turn, the outer lobes divide once again, so that we have a leaf blade consisting, as it were, of six separate lobes, which are again divided into two parts. The width of these lobes, obtained after the final division, is 4—5 mm, with a length of 50 mm and more. In all the positive lobes there run up to four longitudinal, parallel, rather thin veins, between which, at higher magnification, many intermediate, still thinner parallel veinlets are clearly visible. The leaf lobes end wedge-shaped.

Fig. 28 is a leaf blade of smaller dimensions. Its apex is broken off, so that nothing can be said about the form of the endings of the leaf lobes. The width of the segments is 4 mm. Through them pass up to 4 fine veins, between which a delicate intermediate venation is noticeable. The veins do not branch dichotomously. This form very closely approaches the leaf blade figured by A. Krishtofovich in pl. II, fig. I, and referred by him to Ginkgo sibirica Hr. But he himself doubts the correctness of such a determination and considers it quite possible to refer it to Baiera gracilis Bunb.; he does not do so only because of the indistinctness of the venation and the absence of the ends of the leaf lobes. Our specimens are characterized by rather good preservation and make it possible to establish their belonging to Baiera gracilis Bunb., since in external appearance they are entirely identical with the French specimens.

Baiera sp.

Figs. 29—30.

The leaf consists of only two lobes of a peculiar outline (Fig. 29). The left lobe is 11 cm long and at its base reaches 5 mm; then it gradually widens and in its broadest part has a width of 10 mm. Closer to the base, 8 longitudinal, rather sharp veins are visible. In the broad part their number increases, and there are already 10—12, although branching cannot be detected in this area. The lobe ends comparatively bluntly. The right lobe has a strange ending. At first glance it seems that the leaf ends needle-like, but on more attentive examination we see that this shape seems to have resulted from damage to the leaf lobe. In another specimen (Fig. 30) the ends of the lobes are broken off. These specimens could have been referred to Baiera longi-

folia Pom., but the peculiar form of the leaf blades compels one to proceed more cautiously and to define it as Baiera sp.

Locality: Right bank of the Angara River near Strekalovsky Island; Andreevsky mine.

Genus Czekanowskia Heer.

The genus Czekanowskia Hr. was established by Professor O. Heer in 1876 on the basis of a large number of specimens originating from Eastern Siberia and the Amur. In distinguishing the new genus, he himself had doubts as to its position in the systematics of extinct types. The fasciculate arrangement of the thin segments of the leaf, resembling bundles of needles, led Heer to regard them as belonging to the conifers, but the dichotomous branching of the narrow blades did not allow the question to be resolved in such a simple form, and he approached the choice of a place in paleophytological systematics more cautiously. Moreover, in the specimens at his disposal, the leaf segments were covered by certain formations of irregular shape, situated on the leaf blades without any definite order—now very close to the base, now at their end, now rounded and irregular in form—representing, as it were, a sleeve constricted and divided into separate joints, always of unequal size. Noting this irregularity in their arrangement, Prof. Heer referred these formations to accidental fungal growths, supposing that the fallen leaves had long remained in water, where they were apparently covered by parasitic fungi. Similar formations are not observed on specimens from the Cheremkhovo coal-bearing basin, although the number of specimens reaches several hundred. This fact serves as confirmation of the accidental nature of such formations.

Until very recently, the genus Czekanowskia Hr. was either placed in the group Ginkgoales or was considered to belong to the group of conifers. At present we seem already to have sufficient data to place it more definitely in the group Ginkgoales, and the considerations that permit this I shall give somewhat later.

The narrow leaf blade, by a deep sinus reaching to the very base, is divided into two parts, which in turn branch dichotomously at a rather acute angle. Such branching sometimes reaches five times, and as a result the leaf blade represents a fan-shaped bundle of very narrow leaf blades, no more than 2.5 mm wide. The length of the leaf blade is usually 150–180 mm. The venation consists, as it were, of one median, rather coarse vein. But in reality, if one examines it under a microscope, it proves to be a median elevation of the leaf, bordered on the sides by two longitudinal, rather sharp nerves and entirely covered with thin parallel veins, such as we observed also in the genus Baiera F. Br. Sometimes there are two such elevations, and their structure remains completely analogous.

The leaf petiole was absent, and the leaves were situated at the ends of short branches, where the stipules were also located. Sometimes the leaves fell off together with the short branch, creating the impression of the small cone that we sometimes observe at the base of the leaf in impressions. Prof. A. Braun believed that the shedding of leaves in bundles was due in its origin to fungal formations, which affected the short branches not far from the base of the leaf.

O. Heer described flowers and seeds belonging to this genus and found together with the leaves of the genus Czekanowskia Hr. (Flora fossilis arctica, T. IV, V and VI). The seeds were located on a flattened, unbranched stalk and consisted of two rounded, smooth bodies with a weak me—

with a longitudinal striation. On the inner side they are flat, on the outer side somewhat convex.

The genus Czekanowskia Hr. is very widely distributed in the Jurassic deposits of the whole world, having begun its existence at the end of the Triassic period and ended it at the beginning of the Cretaceous.

Czekanowskia rigida Heer.

Fig. 31—42.

Czekanowskia rigida Heer. Heer (²⁴), p. 70, pl. V, figs. 8—11; pl. VI, fig. 7; pl. X, figs. 2-a.
1911. Czekanowskia rigida A. Seward. (¹⁵) P. 20. Pl. IV, fig. 46.
1911. Czekanowskia rigida A. Seward and H. Hamshaw Thomas (¹⁷). P. 9. Pl. 2, fig. 14.
1910. Czekanowskia rigida A. Krystofovich. (⁴). P. 14. Pl. III, fig. 6.
1905. Czekanowskia Murrayana Sew. F. Krasser (²⁶). P. 25. Pl. III, figs. 6, 7 and 8.
1910. Czekanowskia rigida F. Krasser. (²⁷) P. 10. Pl. II, figs. 7, 8.
1876. Czekanowskia setacea Heer. (²¹). P. 68. Pl. V, figs. 1—7; pl. VI, figs. 1—6; pl. X, fig. 11; pl. 12, figs. 5-в; pl. XII, fig. 10-с.
1910. Czekanowskia setacea A. Krystofovich. (⁴). P. 14. Pl. III, fig. 7.
1878. Czekanowskia setacea Heer. O. (²⁴). P. 26. Pl. VI, figs. 9—12.
1880. Czekanowskia setacea Heer. O. (²³). P. 18. Pl. I, figs. 2-в; pl. V, figs. 3-д; pl. VI, figs. 13—16.

Representatives of the genus Czekanowskia Heer enjoy the widest distribution in the Jurassic deposits of the southeastern part of the Irkutsk coal-bearing basin. The beds of argillite lying in the roof of the Little and Main coal seams are literally filled with fragments of its leaves. (Impressions of complete leaf blades with the base and short shoots are encountered comparatively rarely.) The best and most characteristic of them are presented in the plates.

In fig. 31 the leaf blade is represented rather fully. The base of the bundle with short shoots and the branch on which the leaves were situated is visible. The length of the leaf bundle is 100 mm, with the very tips broken off. The branching of the individual lobes reaches five times. Their width on average is 1 mm. But some lobes on this same leaf blade have a width of 0.5 mm, while others, the broadest, reach 1.25 mm. Thus the variation in the width of the lobes within a single leaf blade ranges from 0.5 to 1.25 mm. The nervation is faintly visible. Nevertheless, under the microscope one can see a fine striation, in places covering the median elevation.

The specimen shown in fig. 33 differs somewhat from the preceding one by having narrower leaf blades, the width of which varies from 0.5 mm to 0.75 mm. Their dichotomous branching reaches four times. The length of the whole leaf blade is 80 mm. The nervation is clearly visible. The median thickening or elevation occupies one third of the leaf lobe and is bordered on the sides by rather coarse veins; the space between them is entirely covered with very fine parallel veinlets. Fig. 32 represents a rather rare bundle of leaf lobes, 1 mm wide, but such specimens, as well as those shown in figs. 35, 37, 38, and 42, are comparatively few in number. The greater part of them is represented by fragments

leaf blades in the form of impressions, shown in Figs. 34, 37, 39, and 41, with the most varied average width of the leaf lobes. Figs. 35, 37, 38, 39, 41, and 42 represent leaves of rather good preservation. With the exception of the form depicted in Fig. 37, all these specimens may be assigned to the species Czekanowskia setacea Heer. It should be noted that by many authors, including Prof. O. Heer himself, this species is distinguished from Czekanowskia rigida Hr. by narrower leaf lobes, reaching barely 0.5 mm in width and in many cases even considerably less. Thus we note that the species Czekanowskia setacea Hr. represents the highest degree of dissection of the leaf blade into the finest hair-like separate leaf parts, whose dichotomous branching also approaches the maximum, reaching up to five times.

In the collection under study there is a certain number of leaf blades in which the width of individual lobes, in one and the same form, fluctuates on both sides of the boundary separating the two species. Understandably, in such cases it is difficult, and even impossible, to decide in a more definite form to which species a given impression belongs. In these cases even secondary characters do not extricate us from the difficulty. Among the secondary characters is the number of dichotomous branchings of the leaf lobes. But in order to know this number, we must have impressions of entire leaf blades, which is a matter of fortunate chance; such remains, owing to the rather fragile structure of the leaf, occur in very limited numbers. Usually we have at our disposal only fragments of leaf blades, or even fragments of individual leaf lobes with dichotomous division and typical venation. Moreover, even in the case where we have before us a whole series of impressions of excellently preserved complete leaf blades, we shall then see, as in representatives of the genus Baiera F. Br., that the outer lobes will have more forked branchings than the inner ones. Thus, for example, on our own specimens I was able to observe fluctuations in the number of forked divisions in one and the same blade in the following form: the outer lobes underwent fivefold division, whereas the inner ones underwent three- to fourfold division. Thus we see that this character too cannot be decisive in separating two closely related species from each other.

A careful examination of a very large number of leaf lobes involuntarily leads us to the conclusion that, apparently, in the present case we are dealing with different modifications of the leaves of one species, Czekanowskia rigida Heer, and, of course, in order to decide the question whether we have representatives of one natural, quite independent group or of two different, closely allied forms, we must turn to the method of variation statistics, by means of which questions of this kind can be easily resolved. But before applying this method to the solution of the present question, I think it will be appropriate to acquaint the reader with the method itself, for which purpose it is necessary to digress for a time into the field of biological knowledge.

If we consider two generations of organisms related to one another as parents to their children, two phenomena will strike us. First, among the offspring of one and the same pair we shall not find representatives indistinguishable from one another: in this lies the essence of the variability of organisms. On the other hand, all children are more or less similar to their parents: in this lies the essence of heredity.

The phenomena of individual variability are characteristic of all organisms without exception and gradually pass into another, broader phenomenon—evolution. Three kinds of individual variability are known: 1. simple variations or modifications, 2. mutations, and 3. combinations.

We shall dwell on the consideration of the first type of individual variability, namely on simple variations or modifications. Phenomena of this kind are known to everyone. It is enough to take a considerable number of individuals of the animal or plant kingdom and to pay attention to some one character, for example, coloration, size, weight, etc., in order to see that the given character will not remain a constant magnitude, but will vary within the most diverse limits. As the final result of the examination, we can arrange all individuals in a series according to the change of some one feature. This series will begin with forms with the least development of the given character and end with forms with the maximal development of this feature. The extreme members of such a series will be connected with one another by gradual transitions. Such a series of variability is called a variation series. To illustrate this series the following examples may be given (19, 20).

Figure 43 presents a schematic representation of the variation series of cherry-laurel leaves according to their length.

Quetelet, studying the variability of the height of 25,000 North American recruits, noted that the main mass of them had an average height of 67 inches and that deviations from this mean in either direction occurred the more rarely, the greater the magnitude of the deviations, as is seen in the following table:

The brain weight of 416 persons in another study varied within the following limits and gave this series:

Let us take one more series of variability—the length of 400 bean seeds:

In order to depict graphically, for example, the last variation series, it is necessary to proceed as follows. On the horizontal axis (Fig. 44) we shall mark, at equal distances from one another, points corresponding to the lengths 8, 9, 10, and 16 mm. At these points we shall erect perpendiculars and on them, at a definite scale, lay off the numbers of variants of each class. Then, by joining the tops of the perpendiculars with straight lines, we shall obtain a curve that clearly depicts our variation series. Such a curve is called a curve of variability or a variation curve. It is clear that the more variants we have at our disposal, the closer these curves will approach the ideal curve of variability (Fig. 45). Careful examination of the numbers corresponding to the number of variants of each class leads to the conclusion that the ideal curve very closely approaches to

curve constructed according to the binomial formula \((a+b)^n\). This regularity, in the distribution of individual variability first noted and scientifically substantiated by Quetelet, received the name of Quetelet’s law of variability.

In addition to establishing a variation series and constructing a variation curve, it is also necessary to know the mean magnitude of the given character and the degree of possible deviation from it. It would seem that the mean magnitude can be determined very simply, by dividing the length of the variation series in half. Determining in this way the mean magnitude of the series of bay-leaf leaves, we obtain it as equal to 100 mm. But in reality the mean magnitude is a length of 105 mm, i.e., we become convinced that it depends not only on the length of the variation series, but also on the number of variants in each class. It is clear that the class closest to the mean magnitude will be the one to which the greatest number of variants belongs, or, more precisely, that place on the curve where it has the greatest ordinate. However, this determination cannot be considered absolutely exact, since we will not always have so large a number of individuals that our curve can come very close to the ideal curve of variability, and therefore, in those cases when we need to know more precisely the mean magnitude of a series, we shall have to turn to mathematics to calculate the arithmetic mean, for which it is necessary to take the number of individuals of each class, multiply it by the magnitude of the class, add the sum of the products obtained, and divide it by the number of all variants.

As for the calculation of the mean deviation from the mean magnitude, or, more precisely, the mean square deviation, it is calculated by the following formula:

\[ L = \pm \sqrt{\frac{\sum p a^2}{n}} \]

where \(L\) is the mean deviation;
\(a\) is the deviation of each class from the mean magnitude,
\(p\) is the number of variants of each class,
\(n\) is the number of all variants.

If the index of deviation calculated by this formula is expressed as a percentage of the arithmetic mean, we obtain the so-called coefficient of variability, or coefficient of variation. With the aid of coefficients we can already compare the variability of the most diverse characters.

With regard to the causes that produce modification variability in organisms, it may be said that they naturally fall into two groups: causes, so to speak, of an internal character, hidden in the organism itself, and external causes, conditioned by external influences of the environment. There is no need to say much about the former, since it is clear to everyone that variability is above all caused by the internal properties of the organism itself. It is known that one species is comparatively little variable, while another, standing next to it, is distinguished by extreme inconstancy and occurs in the form of a large number of different forms. As an example one may point to our garden plants, among which pansies are distinguished by great variability of characters, while mignonette, on the contrary, is characterized by the greatest stability of them. In the same way, variability is sometimes very different in separate organs of one individual. While one of them varies extremely strongly, another, on the contrary, in the same individual is more or less constant. All these cases can be explained only by assuming that here, above all, certain internal factors lying within the organism itself play an important role.

The variability of organisms under the influence of external conditions is an unquestionable fact, very easily demonstrated wherever investigations are conducted with the aid of precise statistical methods. Thus, Johannsen, over the course of five years, from 1903 to 1907, observed the variability of the average weight of bean seeds originating from a single maternal plant. The average weight of the seeds changed from year to year as follows: in 1903 it was equal to 63 g, in 1904—73 g, in 1905—55 g, in 1906—63 g, and in 1907—74 g.

The influence of external conditions appears still more clearly if one uses coefficients of variation. Studying variability in the larvae of sea urchins, Peter deliberately altered the external conditions, increasing, for example, the salinity of the solution, exposing them to heat or cold, etc., and obtained the following figures for the coefficient of variation:

All these examples once again demonstrate the unquestionable influence of external conditions on the variability of the organism.

It now remains for us to become acquainted with the role played by the statistical method in solving certain special biological questions, chiefly concerning the variability of the organism. We already know that every natural independent group of organisms, i.e. a species, variety, or race, has its own variation curve, and, indeed, in many cases two such curves, each belonging to a distinct species or variety, stand at a considerable distance from one another, i.e. between the two variation series there is a break and they do not touch one another (fig. 46). The situation is somewhat more complicated in the case of so-called transgressive variability, in which two curves, usually belonging to two more closely related forms, overlap one another at their margins (fig. 47). To determine to which species an individual belongs in transgressive variability is impossible without applying the statistical method. But even with the aid of the statistical method, the solution of such problems is very difficult and complex, although for the most part still solvable. For this it is necessary to determine precisely the mean values of the individuals and to calculate the sum of the squares of the deviations from their mean value, which, for its race, species, or variety, will always be the smallest value.

Now, after a brief acquaintance with the procedures of the statistical method, it is already possible to return to our material and try to resolve the question of whether our impressions belong to one species or to two different species.

For this purpose I took 396 leaf blades belonging to the genus Czekanowskia Hr. and originating from one argillite horizon in the roof of the main seam. As a result of the measurements, it proved possible to divide all the leaves into a whole series of classes and to arrange them in the following variation series.

In order to depict graphically the series we obtained, one may proceed in the following manner (fig. 48). On a horizontal line we mark, at equal distances from one another, points corresponding to widths of 0.25, 0.50 mm, and so on. At these points we erect perpendiculars, the length of which, on a definite scale, will correspond exactly to the number of individuals in each class. By joining the tops of these perpendiculars with straight lines, we obtain a broken curve that clearly represents our variation series. The curve shows that before us there is one quite definite natural group, whose mean width of the leaf lobes is equal to 0.75 mm. Now, if we take specimens described from various Jurassic deposits of Eastern Siberia and the Amur region, our curve can be supplemented by a dotted line, and the variation series will become complete. Thus, for example, Prof. O. Heer described specimens from Irkutsk Province with hair-like leaf lobes on one side and specimens with a lobe width of 2–2.5 on the other side. All these specimens were encountered in very limited quantity, which is quite natural, since they represented the most extreme manifestations of the individual variability of the species.

Thus, on the basis of the variation series and variation curve obtained by us, we must say that if the width of the leaf lobes of the genus Czekanowskia Hr. is taken as a character on the basis of which this genus was artificially divided by O. Heer into the species Czekanowskia rigida Hr. and Czekanowskia setacea Hr., then we shall have representatives of a single natural group, or species, whose name must be given with allowance for the mean value of the series. Close to the mean value will be the forms that were described by many authors under the name Czekanowskia rigida Heer and that are found in many parts of the world. As for the species Czekanowskia setacea Hr., we shall consider that the forms described under this name belong to the species Czekanowskia rigida Heer and represent only a more extreme degree of dissection of the leaf blade into separate lobes.

Localities: Right bank of the Angara River near the Volkovsky, Strekalovsky, and Kampaneysky Islands. Kasyanovsky, Grishevskii mines. Andreevskii mine. Pond of the town of Cheremkhovo. Lower reaches of the Irkut River.

Concluding conclusions.

The group Ginkgoales in the Irkutsk coal-bearing basin is represented by the following forms:
1. Ginkgo Huttoni (Sternb.) Heer. 2. Ginkgo digitata Heer. 3. Ginkgo sibirica Heer. 4. Ginkgo sp. 5. Baiera longifolia Pom. sp. 6. Baiera Czekanowskiana Hr. 7. Baiera pulchella Heer. 8. Baiera gracilis Bunb. 9. Baiera sp. 10. Czekanowskia rigida Heer.

For the most part, the species mentioned were described from various Jurassic deposits of Eastern Siberia, with the exception of only one form, Baiera gracilis Bunb., which has been encountered in Siberia for the first time and agrees fairly well with French specimens.

The division of the fossil representatives of the group into separate species is, on the whole, a rather difficult matter and to a high degree artificial.

The stone material, consisting of mass impressions of leaves of the genus Czekanowskia Heer, makes it possible once again to confirm concretely the presence of individual variability in particular representatives of the group and to identify two species, Czekanowskia rigida Hr. and Czekanowskia setacea Hr., as one species. In addition, in studying such a large number of specimens, the thought arose of the possibility of elucidating the systematic position of the genus itself.

The genus Czekanowskia Hr. up to the very most recent days has not had an entirely definite position in the systematics of extinct types of the plant kingdom, and it seems to me that this question can be resolved in a more positive sense only when, in aid of the paleontological record, we make use of one more means at our disposal—namely our reasoning. Everyone knows that a mathematician, knowing only a small segment of a curve, can calculate the regularity of its course and construct the entire curve. In this way astronomers calculate the paths of motion of stars in celestial space. In a similar manner, we too must seek out a certain regularity in the study of fragmentary paleontological material and, already on the basis of the regularities found, construct the path of evolution of organisms. Only by such a path shall we more quickly approach the truth.

The paleontological record gives us the possibility of establishing the relative antiquity of a given group and of its representatives. Thus, we know that the most ancient and primitive representative of the group Ginkgoales is the genus Ginkgo L., which appeared as early as the end of the Carboniferous period. The most narrowly specialized representative from the point of view of modern botany will be the genus Czekanowskia Hr., younger in its distribution, which began its existence at the end of the Triassic period. An intermediate position in the character of its organization and in age is occupied by the genus Baiera F. Br., which appeared at the end of the Permian period or at the beginning of the Triassic. The whole group attains its luxuriant flourishing in the Jurassic, after which its representatives die out comparatively quickly, and only its most primitive member, with very weakly dissected leaves, in the form of the single living species Ginkgo biloba L., has continued its existence to the present day.

If we examine the venation of the leaves of the genera Baiera F. Br. and Czekanowskia Heer, we shall see that almost all their representatives have longitudinal parallel nerves, between which pass intermediate, more numerous and finer veinlets. The number of main nerves is not constant: in the broadly lobed forms of the genus Baiera F. Br. it reaches 12 and more; in forms with narrower divisions of the leaf this number gradually declines to three; in the genus Czekanowskia Heer, which is characterized by the least width of the leaf lobes, reaching at most 2–2.5 mm, the number of longitudinal nerves is equal to three or two.

The degree of dissection of the leaf blade, as a result of which dichotomous branching of the leaf lobes is obtained, shows us that the number of branchings gradually increases from the genus Ginkgo L. to the genus Czekanowskia Hr. Thus, in the leaves of the genus Ginkgo L., we count such

lobes from 1 to 3; in the genus Baiera F. Br. from 2 to 4, and, finally, in the genus Czekanowskia Hr. from 3 to 5.

The leaf petiole has the greatest length in the genus Ginkgo L., a comparatively insignificant length in the genus Baiera F. Br., and is entirely absent in the genus Czekanowskia Heer.

In the first two genera the leaves were situated on short shoots together with bracts and flowers. As for the genus Czekanowskia Hr., its leaves were likewise placed, together with bracts and flowers, on certain enigmatic scaly cones, which in no essential respect differ from the tips of the short shoots even of the modern representative Ginkgo biloba L.

Thus, we arrive at the conclusion that the genus Czekanowskia Heer is the highest member of one phylogenetic branch, Ginkgo—Baiera—Czekanowskia, which arose from the genus Baiera F. Br. through complete reduction of the leaf petiole and a more considerable dissection of the leaf blade into narrower parts, owing to changing external conditions.

As for the age of the Irkutsk coal-bearing basin, it is rather difficult to establish it more precisely on the basis of the study of a single group, considering in general the poor state of knowledge of Mesozoic floras and the complete absence of guide forms; nevertheless, it is possible to make some approximation.

Thus, on the basis of the literature data we can construct curves that graphically represent the history of development of the individual genera of the group Ginkgoales; for this purpose, on a horizontal line we mark points corresponding to different moments in the earth’s history, and on the corresponding verticals—the numbers of specific varieties at each given moment. In order to compare these curves with one another more conveniently, it is necessary to superpose them (fig. 49). The collection studied shows us that in the Irkutsk coal-bearing basin the predominant importance was held by the highest representative of the group Ginkgoales, the genus Czekanowskia Hr., fragments of whose leaves fill the argillite horizons. Apparently, the genus Czekanowskia Hr. is found here in luxuriant development, whereas the genus Ginkgo L. is represented by comparatively few and greatly reduced leaves, which indicates its unquestionable decline. The genus Baiera F. Br. occupies an intermediate position. Such a moment will correspond to the ordinate ab, indicating the Upper Jurassic age of the basin.

Moreover, a preliminary examination of all the stone material, among which there are also fragments of true deciduous trees together with leaves of the genus Czekanowskia Heer, also makes one doubt the Middle Jurassic age of the basin and gives grounds to express a certain supposition of a younger age; this, of course, can be clarified definitively only after processing all the paleophytological material of the Irkutsk coal-bearing basin.

Explanation of Figures.

1–2. Ginkgo Huttoni (Stern) Heer.

3–4. Ginkgo digitata Heer.

5–8. Ginkgo sibirica Heer.

1. Ginkgo sp.

10–15. Baiera longifolia Pom. sp.

16–22. Baiera Czekanowskiana Heer.

23–26. Baiera pulchella Heer.

27–28. Baiera gracilis Bunb.

29–30. Baiera sp.

31–37. Czekanowskia rigida Heer.

38–42. Czekanowskia rigida Heer.

1. Variation series of laurel-cherry leaves. M—mean value.

2. Curve of variation in the length of bean seeds.

3. Ideal variation curve. From Bauer.

4. Curve showing head height in Hyalodaphnia. After Woltereck, from Goldschmidt. M—mean height.

5. Curves showing fluctuations in the number of marginal florets in Chrysanthemum segetum (A) and its race Chrysanthemum grandiflorum (B). From Goldschmidt, after de Vries.

6. Variation curve showing fluctuations in the width of the leaf lobes in Czekanowskia rigida Heer.

7. Diagram of the development of the genera Ginkgo L. (a), Baiera F. Br. (b), and Czekanowskia Heer (c).

Bibliography

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2. Depéret, Ch. Transformations of the Animal World, 1921.

3. Koltsov, N. Lotsy’s views on the evolution of organisms. Priroda, 1915.

4. Krischtafovich, A. Jurassic plants of the Ussuri region. Proceedings of the Geological Committee. New series. Issue 56. 1910.

5. Krischtafovich, A. Jurassic plant remains collected in the Ussuri region by I. V. Wittenburg and along the Zea River by P. V. Ripas and Anert. Yearbook of Geology and Mineralogy of Russia. Vol. XVI, issues 2–4. 1914.

6. Krischtafovich, A. Materials toward the knowledge of the Jurassic flora of the Ussuri region. Proceedings of the Geological and Mineralogical Museum of the Imperial Academy of Sciences. Issue 4. Vol. II. 1916.

7. Krischtafovich, A. Jurassic plants from the Tyrma River, Amur Province, collected by V. Dokturovsky. Proceedings of the Geological and Mineralogical Museum of the Imperial Academy of Sciences. Vol. VIII. 1914.

8. Krischtafovich, A. Plant remains from ore-bearing Corbicula deposits of Transbaikalia. Proceedings of the Mineralogical Society. Second series. Part 51, issue I. 1918.

9. Lesnikova, A. Jurassic plants from the Caucasus. Bulletin of the Geological Committee, 1915.

10. Novopokrovsky, I. Materials toward the knowledge of the Jurassic flora of the valley of the Tyrma River (Amur Province). Geological survey and exploration work along the Siberian Railway. Issue XXXII, 1902.

11. Orzhentsky, R. On the statistical method. Priroda, 1914.

12. Palibin, I. F. Krasser’s work on fossil plants of China and Central Asia. 1902.

13. Reis, O. Fauna of the fish shales of Transbaikalia. Geological survey and exploration work along the Siberian Railway. Issue XXIX. 1910.

14. Seward, A. Jurassic plants of the Caucasus and Turkestan. Proceedings of the Geological Committee. New series. Issue 38. 1907.

15. Seward, A. Jurassic plants from Chinese Dzungaria, collected by Prof. Obruchev. Proceedings of the Geological Committee. New series, issue 75. 1911.

16. Seward, A. Jurassic plants from the Amur region. Proceedings of the Geological Committee. New series. Issue 81. 1912.

17. Seward and N. Thomas. Jurassic plants from Balagansk District, Irkutsk Province. Proceedings of the Geological Committee. New series. Issue 73. 1911.

18. N. Thomas. The Jurassic flora of Kamenka in Izyum District. Proceedings of the Geological Committee. New series. Issue 71. 1911.

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20. Shchelkanovtsev, Ya. Introduction to Evolutionary Doctrine, 1919.

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22. O. Heer. Jura-Pflanzen aus der arctischen Zone Sibiriens. 1878.

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2. Krasser F. Die von W. Obrutschewi in China und Centralasien 1893—94 gesammelten fossilen Pflanzen. 1900.

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9. Zittel K. Handbuch der Paleontologie. II Abteilung.

To the article by V. A. Khakhlov: “The Jurassic group Ginkgoales of the Irkutsk coal-bearing basin.”

Plate I.

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Plate II.

Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10.

Fig. 12.

Plate III.

Fig. 11.

Fig. 13.

Fig. 14.

Fig. 15.

Plate IV.

Fig. 16

Fig. 17.

Fig. 18.

Fig. 20.

Fig. 19.

Plate V.

Fig. 21.

Fig. 22.

Fig. 23.

Fig. 24.

Table VI.

Fig. 25.

Fig. 28.

Fig. 29.

Plate VII.

Fig. 27.

Fig. 30.

Fig. 28.

Plate VIII.

Fig. 31.

Fig. 32.

Fig. 33.

Fig. 34.

Fig. 35.

Plate IX.

Fig. 36.

Fig. 37.

Fig. 38.

Fig. 39.

Fig. 40.

Fig. 42.

Fig. 41.

Pl. X.

Fig. 43.

\(l = 63\) mm.  \(l = 105\) mm.  \(l = 137\) mm.

Fig. 44.

Fig. 45.

Fig. 46.

\(M\)  \(M_1\)

Plate XI.

Fig. 47.
Labels: A, B.
Axis markings: 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29.

Fig. 48.
Axis markings: 0, 0.25, 0.50, 0.75, 1.00, 1.25, 1.50, 1.75.

Fig. 49.
Labels: a, b, \(O\), \(P\), \(T\), \(I_1\), \(I_2\), \(I_9\), \(C_e\), \(T_p\).