Fossil plants, Vol. 3

I. LYGINOPTERIDEAE.

LYGINOPTERIS.

The genus Lyginopteris is selected for the first place in this chapter simply on the ground that we have a fuller knowledge of its morphology than in the case of other types. It is not regarded as the most primitive member of its class. Lyginopteris may be described in a few words as a plant having the habit and to a large extent the anatomical features of a Fern, but differing from existing ferns in the possession of integumented megasporangia or seeds and in the power of secondary growth in thickness by means of a cambium in both stem and root. The seed (Lagenostoma) agrees with those of recent Cycads and Gnetales more closely than with the corresponding organs in Conifers or any other group, while the structure of the secondary wood is practically identical with that of Cycads. The microsporangia occur as groups of small bilocular sporangia, or synangia, at the tips of fertile pinnae of highly compound fronds.

Nomenclature and Historical Summary. In 1866 E. W. Binney[93] of Manchester published a short description of a small petrified stem from the Lower Coal Measures of Lancashire and named it Dadoxylon oldhamium, employing Endlicher’s term Dadoxylon which that author substituted for Pinites as previously used by Witham[94]. Three years later Williamson[95] drew attention to certain features in which Binney’s type differs from the genus Dadoxylon and substituted a new name Dictyoxylon, suggested by the reticulate pitting on the walls of the tracheids. In a subsequent paper Williamson[96] gave a fuller description of Binney’s species and spoke of it as ‘one of the most common plants in the calcareous nodules of the Lower Coal Measures’ of Lancashire and Yorkshire. He connected certain casts of arborescent dimensions with Binney’s type on the ground that the surface-features of the casts are such as would be produced by partially decorticated stems having a hypodermal reticulum of mechanical tissue like that preserved in the small petrified specimen described by Binney (fig. 402). Mr Carruthers called Williamson’s attention to a paper by Mr Gourlie[97] in which the generic name Lyginodendron is instituted for stem-casts identical in surface-features with the fossils figured by Williamson. In spite of the much larger dimensions of the reticulum on the casts described by Gourlie as compared with that in the outer cortex of Binney’s stem, Williamson concluded that Lyginodendron is ‘undoubtedly an inorganic cast of the prosenchymatous layer of the bark of Dictyoxylon.’ It is but fair to add that Williamson was influenced in coming to this conclusion by a discovery by Mr Nield of a piece of a large petrified stem believed to be generically identical with Binney’s type, but subsequently referred to a distinct genus[98], which was comparable in size with the stems responsible for Gourlie’s Lyginodendron casts. The type-specimen of Gourlie’s Lyginodendron Landsburgii[99], from Carboniferous rocks at Stevenston in Ayrshire, Scotland, is represented in fig. 401. The convex areas represent casts of depressions in a reticulum of cortical tissue, originally occupied by comparatively delicate cells, which decayed or shrunk more quickly than the enclosing framework of stronger fibrous elements that remained as a prominent reticulum and produced the depressions bounding the raised portions of the cast. Such a cast would undoubtedly be formed by the stem on which Binney founded his species: the radially disposed bands of thick-walled cells seen in the outer part of the section (fig. 402) are portions of an irregular anastomosing mechanical system, the reticulate arrangement of which is seen in the impression of a rachis of a Lyginopteris frond shown in fig. 405, E, and indicated in the more slender axis reproduced in fig. 404, A, b. This reticulate form of cortical stereome on which Brongniart founded the genus Dictyoxylon[100], a term since applied by Solms-Laubach and other authors to a certain type of cortex not confined to a single genus of plants, occurs also in some Palaeozoic lycopodiaceous stems[101] and in itself cannot be regarded as a safe criterion of botanical affinity. The largest example of Gourlie’s Lyginodendron that has come under my notice is an incomplete sandstone cast from Upper Carboniferous strata near Harrogate reaching a length of 100 cm. and with convex areas 13 cm. long. A similar cast, 36 cm. broad, has recently been figured by Nathorst[102] from the Culm of Spitzbergen, and from the Upper Devonian of Ellesmere Land the same author has described impressions of a cortical reticulum under the name Lyginodendron Sverdrupi[103]. These specimens are interesting as pointing to the former occurrence in the Arctic regions of stems—probably Lepidodendroid—reaching the dimensions of a fairly large tree. As Potonié[104] pointed out, Gourlie’s generic name serves a useful purpose for casts of stems of the type shown in fig. 401 that cannot be assigned to a definite systematic position. The genus was first used for a specimen which has nothing to do with the plant usually spoken of as Lyginodendron oldhamium (Binney). Though loath to give up a name by which Binney’s type has long been known, in spite of its retention in the second volume of this work I feel compelled so far to conform to the recognised principles governing nomenclature as to adopt Potonié’s generic term Lyginopteris.

i. Stem.

The petrified stem on which Binney founded the species was first figured by Dr Arber[105] from a section in the Binney collection in the Sedgwick Museum, Cambridge: this section (13 mm. in diameter) is reproduced in fig. 402. The most striking features are: (i) the pith consisting of an unusually large and irregular group of dark thick-walled parenchyma, (ii) the broad cylinder of manoxylic secondary xylem characterised by multiseriate medullary rays, (iii) the outer cortex composed of dark radially disposed and oblique bands of mechanical tissue separated from one another by partially destroyed and tangentially elongated parenchymatous elements. It is this outer cortex that Williamson aptly compared with the Roman numerals on a clock-face. In the perimedullary region and in contact with the inner edge of the secondary-xylem cylinder are six strands of primary xylem representing the xylem halves of collateral bundles separated from the primary phloem strands by the intervening cylinder of secondary wood. Two of the primary xylem strands in lateral contact are seen in fig. 404, C; the other four occur as separate bundles. Each primary xylem strand contains a small group of spirally thickened protoxylem elements (px) associated with a few parenchymatous cells. The large primary tracheids internal to the protoxylem are characterised by multiseriate bordered pits on their walls, while those external to the protoxylem, which are in contact with the innermost secondary tracheids, have scalariform pitting. The dark patch s (fig. 404, C) is a portion of the large group of sclerenchymatous cells, shown in figs. 402, 403. The perimedullary xylem strands of mesarch structure are the lower portions of leaf-traces and, as Scott points out, ‘each of the bundles surrounding the pith is, in fact, a sympodium, composed of the united lower ends of successive adjacent leaf-traces.’ The larger of the two bundles shown in fig. 404, C, is on the point of passing out to a leaf, while the smaller strand is on its way to a higher level before bending outwards through the secondary wood. Slightly beyond the middle of the secondary xylem there is an arc of narrower tracheids comparable with an incomplete annual ring. Although zones or arcs of narrow tracheids are not uncommon in the wood of Lyginopteris there is no satisfactory evidence of regularly recurring seasonal changes. On the outer face of the secondary wood are a few leaf-trace strands pursuing a vertical course in the pericycle region; but the structure and behaviour of these bundles are more clearly illustrated in the stem reproduced in fig. 403. The tissue between the crushed phloem and pericycle and the outer cortex (fig. 402) consists of radially compressed parenchyma with scattered secretory cells separated from the more internal tissue by a narrow band of periderm formed by a phellogen in the outer part of the pericycle.

A larger and better preserved stem, 3·7 cm. in diameter, is seen in fig. 403. In this stem the pith of parenchyma and scattered sclerenchymatous nests is larger in proportion to the stele than in Binney’s type-specimen. From the inner edge of the secondary xylem several primary xylem-strands project as rounded wedges or tangentially elongated groups where two traces are laterally united in the perimedullary zone. The cylinder of secondary wood is partially interrupted at r by the bending outwards of the stele of an adventitious root cut across transversely as it bends down after emerging from the outer cortical region. In more or less close association with the outer surface of the secondary xylem are four pairs of leaf-trace bundles and one larger trace at d containing two widely separated protoxylem strands and faced externally with an arc of secondary xylem: this is a leaf-trace which shows by the slight constriction on the outer edge of its primary xylem that it is beginning to divide into a pair of equal strands. A precisely similar strand is shown on a larger scale in fig. 404, D. The twin bundles seen at b, fig. 403, represent a divided leaf-trace at a slightly higher level than the partially severed trace at d, and the arcs of secondary xylem are narrower. The appearance of the double leaf-trace at a still higher level is shown at c: the two strands are farther apart and the secondary xylem has almost disappeared, while those at e, nearer their entrance into the leaf-stalk, consist exclusively of primary xylem and phloem. At a the two strands of a leaf-trace, still nearer to the petiole, are inclined towards one another preparatory to reunion after reaching the leaf-stalk. A slender root is seen in transverse section at r′ immediately outside the two leaf-bundles. As Williamson and Scott[106] have pointed out, there are always five leaf-traces beyond the xylem cylinder of a Lyginopteris stem as seen in transverse section, and these traces in the pericycle, separated from one another by ⅔ of the circumference, alternate in position with the lower portions of leaf-traces in the perimedullary region of the same stem. The phyllotaxis is thus seen to be ⅖.

The secondary wood is succeeded by a cambium of normal structure passing gradually into a narrow band of secondary phloem which in well-preserved stems is seen to consist of sieve-tubes and parenchyma with medullary rays rather broader than those in the xylem. Beyond the phloem is the comparatively broad pericycle consisting of parenchyma with nests of sclerenchyma like those in the pith and scattered secretory cells. In the outer layers of the pericycle a phellogen was formed at an early stage in the growth of the plant, producing several layers of secondary tissue, which is regarded as periderm and forms a conspicuous feature in Lyginopteris stems; it appears as a comparatively dark sinuous band where it bends outwards to wrap round the leaf-traces in their almost vertical course through the pericyclic region (fig. 403). The periderm is clearly seen at p close to the crushed secondary phloem of the dividing leaf-trace in fig. 404, D. All the leaf-traces seen in fig. 403 beyond the secondary wood are still within the deep-seated periderm and, as Williamson and Scott showed, each leaf-trace after emerging from the secondary wood remains in the pericycle-zone for a length of five internodes as it very gradually inclines outwards. Once free from this region the twin bundles bend more sharply towards the petiole. Stated briefly, the history of each leaf-trace from the perimedullary region to the leaf-base is as follows: at the outer edge of the pith a single trace consists of a mesarch xylem bundle with one protoxylem strand; it passes vertically through five internodes and then bends out through a foliar gap in the xylem-cylinder, and the primary tracheids receive additions from the cambium of the stele on their outer face. As the trace leaves the secondary xylem it bends upwards and, as seen at d, fig. 403, begins to divide into twin bundles. As the trace passes higher the bisection of the protoxylem and metaxylem is completed and the secondary xylem-arcs are gradually lost until the separate strands of each pair are reduced to single mesarch bundles composed wholly of primary tracheids. As the trace bends outwards through the cortex the phloem gradually encircles each xylem-strand until a concentric structure is substituted for the collateral disposition of the conducting tissue. At the same time the protoxylem strands divide and occupy a position near the inner edge of the metaxylem. On reaching the petiole or after passing some distance up the axis of the frond, the twin bundles unite and usually form a V-shaped vascular strand (figs. 404, E; 405, A). The single meristele subsequently divides into two equal portions preparatory to the bifurcation of the petiole (fig. 406).

The inner cortex, consisting of parenchymatous tissue and many secretory cells with an occasional group of sclerenchymatous elements in place of the abundant nests of this tissue in the pericycle, has been invaded in the stem shown in fig. 403 by numerous rootlets of Stigmaria and Lyginopteris, some of which are seen interrupting the continuity of the outer cortex. The greater width of the cortical region at f, fig. 403, is due to the decurrent base of a petiole the meristele of which is not included in the section. The lighter and broader bands between the cross-sections of the stereome-network in the outer cortex are occupied by remains of tangentially stretched parenchymatous cells, and beyond this zone in a younger stem there are a few layers of parenchyma forming the superficial tissue, but there appears to be no well-defined epidermal layer.

Young stems have been recognised in which there is very little secondary xylem and phloem: in these the stereome bands in the outer cortex are closer together than in the stretched hypodermal tissue of older shoots and the scattered sclerous nests are represented by unthickened cells. In addition to young stems Williamson and Scott described a distinct type of small stem in which the primary xylem forms an almost complete ring[107] comparable with the primary xylem of some adult Sigillarian stems (vol. ii. p. 220) but distinguished by its mesarch structure and by the reticulate pitting of the centripetal xylem.

A characteristic feature of the stem is the occurrence of emergences from the outer cortex which have the structure either of spinous processes, broadly linear or flask-shaped, or of stalked glands[108]. A portion of a glandular emergence is shown in fig. 405, B: the group of small cells immediately below the blunt apex is in this instance still intact though showing signs of disorganisation in the centre; but in many cases the secretory tissue has not been preserved and the head of the emergence is occupied by a space. A single stoma is seen at s in longitudinal section. Further reference to the emergences is made in the description of the leaf.

It occasionally happens that a meristematic layer is formed in the parenchymatous tissue immediately internal to some of the perimedullary xylem strands of a Lyginopteris stem from which either secondary parenchyma is produced or a zone of secondary xylem and phloem, the phloem facing the centre of the pith. An example of such internal xylem was figured by Williamson[109] and similar occurrences are more fully dealt with by Williamson and Scott[110] who consider that the perimedullary cambium may represent an internal extension through a leaf-gap of the normal cambial cylinder. In the stem represented in fig. 403 there are two perimedullary xylem strands to the left of the bottom of the V-shaped gap in the secondary xylem-cylinder, r, and on the inner face of one of these, as shown in fig. 405, C, there is a narrow arc of internal secondary xylem, c, between the xylem-strand and the outer edge of one of the sclerous nests. The sporadic occurrence of arcs of inversely orientated secondary vascular tissue affords an interesting parallel with a similar morphological feature in some recent Dicotyledonous genera such as Tecoma and Iodes. As Williamson and Scott point out, this similarity affords ‘a striking warning against the indiscriminate use of even conspicuous anatomical characters[111].’ While admitting the necessity of guarding against the danger of attaching importance to occasional and abnormal characters they may have some significance as collateral evidence in comparisons of different types of stems. It is conceivable that these anomalous arcs of secondary tissue on the inner side of the primary xylem strands may, as Worsdell[112] maintains, be reversions to an ancestral character and in this sense comparable with the strands of inverted vascular tissue in some recent Cycadean stems. The question of relationship of Lyginopteris and allied types to recent Cycads and the Palaeozoic Medulloseae is considered in a later chapter.

In 1902 Lomax[113] described two branching specimens of Lyginopteris, and more recently two others have been discovered at a locality near Bacup in Lancashire which have been thoroughly investigated by Miss Brenchley[114] who constructed models from drawings of serial sections[115]. One specimen shows six leaf-bases in a length of 4¾ inches and branches spring from the axils of five of them: some of the branches show secondary ramifications. The phyllotaxis of the leaf-bases on a branch is always the reverse of that on the main stem, a divergence to which no parallel was found in a selection of trees and shrubs examined by Miss Brenchley. The secondary wood of the stem swells below the point of exit of a branch and frequently a fairly large amount of wood occurs in the pith when a branch is given off: this anomalous wood may help to close the branch-gap.

ii. Leaf.

In his account of Lyginopteris stems published in 1873 Williamson[116] suggested that the vascular bundles met with outside the xylem-cylinder might be the leaf-traces of large fronds and expressed the opinion that the ‘stems or petioles’ previously described by him under the generic name Edraxylon might belong to Lyginopteris. A year later he substituted the name Rachiopteris aspera for the petioles previously referred to Edraxylon and inclined to the view that this type of Rachiopteris may be the petiole of the Carboniferous fronds known as Sphenopteris Hoeninghausi Brongn., an inference based to a large extent on the occurrence of emergences on Rachiopteris aspera (fig. 404, E) preserved as petrifactions like those on impressions of Sphenopteris Hoeninghausi as figured by Brongniart (figs. 404, A; 405, D, D’). In 1890 Williamson was able to demonstrate the truth of the surmise that Rachiopteris aspera and Lyginopteris oldhamia are respectively the petiole and stem of the same plant, which he believed to be an arborescent fern[117]. The petioles of Lyginopteris fronds, which may reach a diameter of 1 cm., are attached by a broad base to the stem, and as already suggested by the number of internodes traversed by each leaf-trace, the leaves are comparatively far apart. A transverse section of a petiole is shown diagrammatically in fig. 405 A. The hypodermal stereome is a prominent feature, but the narrow radial plates of the stem-cortex tend to be replaced in the rachis by broader and confluent masses of strengthening elements: the upper surface of the petiole is slightly grooved. Glandular and spinous emergences are often very abundant, as in the section reproduced in fig. 404, E. A glandular emergence is seen at a in fig. 405, A. The spinous emergences may be compared with those of Davallia (Odontosoria) aculeata[118], a West Indian fern of climbing habit and with the prickles on Hemitelia and other recent Cyatheaceous fronds[119], while capitate glands, though simpler than those of Lyginopteris, occur on the leaf-stalks of some recent Polypodiaceous species[120]. The concentric meristele may consist in the lower part of the petiole of two separate and slightly curved strands like those seen in fig. 404, E, m: sooner or later the two strands unite to form a wide-open V or a W-shaped bundle with several slightly internal protoxylem groups close to the lower edge. The two sections represented in fig. 406, A and B show the gradual divergence of the two meristeles of a petiole as they approach the level where it divides into two equal branches, a characteristic feature of Sphenopteris Hoeninghausi and allied fronds. At a lower level than that represented in fig. 406 the vascular strand of the petiole would have the form of a W as figured by Williamson in one of his earlier memoirs[121]. The phloem with scattered secretory sacs and the adjacent tissue of the leaf-stalk are occasionally preserved in wonderful perfection[122]. No endodermis has been recognised. Sclerous nests are scattered in the ground-tissue as are also secretory sacs (figs. 404, E; 405, A). A small root r has penetrated the parenchyma of the rachis shown in fig. 405, A.

Sphenopteris Hoeninghausi Brongn.[123], founded on material from English Coal Measures, was regarded by Williamson as the foliage of Lyginopteris chiefly on the ground of the occurrence of emergences on the axes (figs. 404, A, B) and laminae of the impressions like those on the petrified stems, and this comparison received support from the resemblance of the fragments of pinnules associated with Lyginopteris and its petioles in the calcareous nodules to the leaflets of Brongniart’s type. This identification is supported by subsequent work. The quadripinnate fronds, which attain a considerable size, resemble those of recent species of Davallia and other ferns, but the forking of the rachis and branches of the frond is a striking feature: the pinnae may reach a length of 15 cm.[124] The portion of carbonised rachis shown in fig. 405, E, reveals the existence of a hypodermal reticulum like that in the outer cortex of a Lyginopteris stem and the same feature is seen in the more slender axis represented in fig. 404, A, at b.[125] The pinnules are usually deeply lobed and the segments may be comparatively broad and blunt or narrow[126] (fig. 290, C, vol. ii. p. 399; fig. 404, A, a, B; fig. 405, D’). The lamina has a well marked dorsiventral structure: the palisade-tissue next the upper surface is separated from the epidermis by small hypodermal cells, possibly functioning as a water-storage layer, and the central part of the mesophyll consists of loose aerenchyma: the veins are collateral as in recent ferns and stomata occur in the lower epidermis. Emergences are seen both on impressions (fig. 405, D’) and on petrified specimens. A striking feature of the pinnules is the rounded surface caused by the revolute edge of the lamina as seen in the section reproduced in fig. 404, B. This character coupled with the occasional occurrence of groups of short tracheal elements at the termination of the veins denotes a tendency to a xerophilous habit.

On the strength of a very close resemblance between Sphenopteris Hoeninghausi and Calymmatotheca Stangeri (fig. 408, E, F)—characterised by fertile pinnules bearing stellate groups of small linear valves, regarded by Stur as the open lobes of an indusium—Zeiller included Brongniart’s type in the genus Calymmatotheca. The resemblance in general habit between the two species extends to the presence in their rachises of the Dictyoxylon form of cortex[127]. The view formerly held by some authors that the valve-like appendages to the fertile segments of Calymmatotheca are sporangia is incorrect: a re-examination of Stur’s specimen (fig. 408, E, F) has confirmed the original description[128]. The stellate lobes are now regarded as portions of a cupular investment of a seed similar to Lagenostoma Lomaxi, the female reproductive apparatus of Lyginopteris oldhamia. The axes of the fertile pinnae bear small thorn-like emergences probably identical with those on the cupule of Lagenostoma and on the petioles of Lyginopteris oldhamia. It was stated in vol. ii that the fronds known as Sphenopteris Linkii (Goepp.) represent, with other closely allied forms, leaves belonging to Heterangium stems. This statement was based on a misconception: the rachis of Sphenopteris Linkii, as I have satisfied myself by an examination of impressions shown to me by Dr Kidston, exhibits the reticulate pattern characteristic of Lyginopteris and not the transverse ribs characteristic of Heterangium.

It is not an easy task even for those most familiar with Carboniferous fronds to distinguish clearly between species agreeing generally with Sphenopteris Hoeninghausi, a species regarded by some authors as the type of a group of very similar and closely allied forms all of which were probably borne on stems referable to the genus Lyginopteris. The species Lyginopteris oldhamia as generally understood probably includes more than one specific type, and it is safe to assert that in the Carboniferous period Lyginopteris was represented by several forms characterised by highly compound fronds with forked rachises like Sphenopteris Linkii, S. Hoeninghausi, and others. The features characteristic of fronds included in the Sphenopteris Hoeninghausi group have recently been described by Gothan[129]. Stur’s generic name Calymmatotheca originally applied to the species C. Stangeri was applied to Sphenopteris Hoeninghausi by Zeiller, and although the fronds of the latter type have not been found with fertile appendages of the Calymmatotheca type there can be no doubt as to the generic identity of these, barely distinguishable, species both of which belong to stems of Lyginopteris. Prof. Johnson[130] has recently described some impressions from the Coal Measures of Ireland, which he refers to S. Hoeninghausi, bearing stellate groups of lobes like those of Calymmatotheca, and in one case he describes a seed in the middle of the carbonised remains of a stellate group of cupular segments. An examination of the specimen in Dublin convinced me that there is no satisfactory evidence of the seed-nature of the appearance on the rock believed by Johnson to be an elliptical Lagenostoma-like seed. The actual attachment of the stellate lobes to the pinnae of the frond is not clearly demonstrated.

iii. Microsporangia.

In 1905 Kidston[131] announced the discovery of microsporangia on fronds of Lyginopteris: he described specimens from the Coal Measures of Dudley identified by him with Sphenopteris Hoeninghausi showing sterile and fertile pinnae in organic connexion. The fertile pinnules (fig. 407, B) are slightly expanded distally into an oval limb about 2 mm. long bearing 6 to 7 bilocular fusiform microsporangia 3 mm. long and 1·5 mm. broad: in the immature condition the sorus is hemispherical, the summit being formed of the incurved apices of the sporangia. At maturity the sporangia spread out, the sorus assuming the form of an epaulet. Fig. 408, H, shows a sorus in transverse section and in fig. 408, G, the limb and two pendulous sporangia are shown. The microspores, 50–70 µ in diameter, are studded with numerous blunt spines and each spore shows a triradiate ridge. The section reproduced in fig. 407, A, from the Coal Measures of Oldham is probably a bilocular sporangium of the same type as those described by Kidston from Dudley. Dr Kidston[132] describes a second type of microsporangial sorus as Crossotheca Hughesiana which agrees closely with C. Hoeninghausi, but the fertile segments are not associated with any sterile pinnae. The generic name Crossotheca, founded by Zeiller[133] in 1883, was substituted for Sphenopteris on the ground that Brongniart’s species S. Hoeninghausi is shown to possess sporangia of the Crossotheca type. If Kidston’s specific determination is correct, his discovery demonstrates that Lyginopteris fronds bore microsporangia having the characters of Crossotheca, a type characteristic of several Carboniferous species belonging both to the form-genera Sphenopteris and Pecopteris. Reference has already been made to the difficulty of distinguishing between impressions of fronds of the Sphenopteris Hoeninghausi group, a difficulty that is illustrated by Dr Gothan’s statement[134] that the Dudley specimens of Crossotheca are not identical in the character of the sterile pinnules with Sphenopteris Hoeninghausi. An examination of Dr Kidston’s specimens led me to agree with his determination; but, it may be asked, have we any evidence of the association with Lyginopteris fronds of sporangia other than those of the Crossotheca type? Prof. Chodat[135] believes that certain petrified fragments of pinnules occasionally met with in the calcareous nodules bearing sessile and apparently annulate sporangia belong to Lyginopteris fronds. These sporangia appear to be identical with those named by Scott Pteridotheca Butterworthi[136] and regarded by him as filicean sporangia that cannot be referred to any known Carboniferous genus. The piece of lamina bearing an empty sporangium, which may or may not have possessed an annulus, reproduced in fig. 407, C, occurs in association with the larger specimen shown in fig. 404, B, and it would seem not unreasonable to regard both as parts of the same frond, namely a frond of Lyginopteris. As Prof. Weiss[137] points out, the accurate determination of small pieces of petrified pinnules is exceedingly difficult and without more decisive evidence we are hardly justified in asserting that the sporangia figured by Chodat and Scott and that shown in fig. 407 belong to the genus Lyginopteris. Although the available data appear to favour the view generally held that Kidston’s conclusion is correct additional evidence would be welcome.