Fossil plants, Vol. 1

This species is chosen as an example of a French Equisetites of Permian age. It was recently founded by Zeiller[504] on some specimens of imperfect leaf-sheaths, and defined as follows:—

Sheaths spreading, erect, formed of numerous uninerved coherent leaves, convex on the dorsal surface, spatulate in form, 5–6 cm. in length and 2–3 mm. broad at the base, and 5–10 mm. broad at the apex, rounded at the distal end.

The specimen shown in fig. 58, A, represents part of a flattened sheath, the narrower crenulated end being the base of the sheath. The limits of the coherent segments and the position of the veins are clearly marked. Zeiller’s description accurately represents the character of the sheaths. They agree closely with an Equisetaceous leaf-sheath, but as I have already pointed out, we cannot feel certain that sheaths of this kind were not originally attached to a Calamite stem.

The portion of a leaf-sheath and a diaphragm represented in fig. 57, B, agrees closely with Zeiller’s examples. This specimen is from the English Coal-Measures, but it is not advisable to attempt any specific diagnosis on such fragmentary material. It is questionable, indeed, if these detached fossil leaf-sheaths should be designated by specific names. Another similar form of sheath, hardly distinguishable from Zeiller’s species, has recently been described by Potonié from the Permian (Rothliegende) of Thuringia.

The sheaths consist of linear segments fused laterally as in Equisetum. In some specimens the component parts of the sheath are more or less separate from one another, and in this form they are apparently identical with the leaves of Calamites (Calamitina) varians, Sternb. The example shown in fig. 58, E is probably a young leaf-sheath; the segments are fused, and each is traversed by a single vein represented by a dark line in the figure. The regular crenulated lower margin is the base of the sheath, and corresponds to the upper portion of fig. 58, A. This species affords, therefore, an interesting illustration of the difficulty of separating Equisetites leaves from those of true Calamites. Potonié has suggested that the leaf-sheath of a young Calamite might well be split up into distinct linear segments as the result of the increase in girth of the stem.

Other Palaeozoic species of Equisetites have been recorded, but with one exception these need not be dealt with, as they do not add anything to our knowledge of botanical importance. The specimen described in the Flore de Commentry as Equisetites Monyi, by Renault and Zeiller[506], differs from most of the other Palaeozoic species of Equisetites, in the fact that we have a stem with short internodes bearing a leaf-sheath at each node divided into comparatively long and distinct teeth. This species presents a close agreement with specimens of Calamitina, but Renault and Zeiller consider that it is generically distinct. They suggest that the English species, originally described and figured by Lindley and Hutton[507] as Hippurites gigantea, and now usually spoken of as Calamitina, should be named Equisetites. It would probably be better to adopt the name Calamitina for the French species. The type-specimen of this species is in the Natural History Museum, Paris.

When we pass from the Permian to the Triassic period, we find large casts of very modern-looking Equisetaceous stems which must clearly be referred to the genus Equisetites. The portion of a stem represented in fig. 59 known as Equisetites platyodon Brongn.[508] affords an example of a Triassic Equisetaceous stem with a clearly preserved leaf-sheath. The stem measures about 6 cm. in diameter. One of the oldest known Triassic species is Equisetites Mougeoti[509] (Brongn.) from the Bunter series of the Vosges.

The Keuper species E. arenaceus is, however, more completely known. The specimens referred to this species are very striking fossils; they agree in all external characters with recent Horse-tails but greatly exceed them in dimensions.

This plant has been found in the Triassic rocks of various parts of Germany and France; it occurs in the Lettenkohl group (Lower Keuper), as well as in the Middle Keuper of Stuttgart and elsewhere. The species may be defined as follows:—

Rhizome from 8–14 cm. in diameter, with short internodes, bearing lateral ovate tubers. Aerial shoots from 4–12 cm. in diameter, bearing whorls of branches, and leaf-sheaths made up of 110–120 coherent uni-nerved linear segments terminating in an apical lanceolate tooth. Strobili oval, consisting of crowded sporangiophores with pentagonal and hexagonal peltate terminations.

The casts of branches, rhizomes, tubers, buds and cones enable us to form a fairly exact estimate of the size and general appearance of this largest fossil Horse-tail. The Strassburg Museum contains many good examples of this species, and a few specimens may be seen in the British Museum. In the École des Mines, Paris, there are some exceptionally clear impressions of cones of this species from a lignite mine in the Vosges.

It is estimated that the plant reached a height of 8 to 10 meters, about equal to that of the tallest recent species of Equisetum, but in the diameter of the stems the Triassic plant far exceeded any existing species.

It is interesting to determine as far as possible, in the absence of petrified specimens, if this Keuper species increased in girth by means of a cambium. There are occasionally found sandstone casts of the pith-cavity which present an appearance very similar to that of Calamitean medullary casts[510]. The nodes are marked by comparatively deep constrictions, which probably represent the projecting nodal wood. The surface of the casts is traversed by regular ridges and grooves as in an ordinary Calamite, and it is probable that in Equisetites arenaceus, as in Calamites, these surface-features are the impression of the inner face of a cylinder of secondary wood (cf. p. 310). Excellent figures of this species of Equisetites are given by Schimper in his Atlas of fossil plants[511], also by Schimper and Koechlin-Schlumberger[512], and by Schoenlein and Schenk[513].

This species, which is by far the best known British Equisetites, was founded by Brongniart[514] on some specimens from the Lower Oolite beds of the Yorkshire coast. Casts of stems are familiar to those who have collected fossils on the coast between Whitby and Scarborough; they are often found in an erect position in the sandstone, and are usually described as occurring in the actual place of growth. As previously pointed out (p. 72), such stems have generally been deposited by water, and have assumed a vertical position (fig. 11). Young and Bird[515] figured a specimen of this species in 1822, and in view of its striking resemblance to the sugar-cane, they regarded the fossil as being of the same family as Saccharum officinarum, if not specifically identical.

A specimen was described by König[516] in 1829, from the Lower Oolite rocks of Brora in the north of Scotland under the name of Oncylogonatum carbonarium, but Brongniart[517] pointed out its identity with the English species Equisetites columnaris.

Our acquaintance with this species is practically limited to the casts of stems. A typical stem of E. columnaris measures 3 to 6 cm. in diameter and has fairly long internodes. The largest stem in the British Museum collection has internodes about 14 cm. long and a diameter of about 5 cm. In some cases the stem casts show irregular lateral projections in the neighbourhood of a node, but there is no evidence that the aerial shoots of this species gave off verticils of branches. In habit E. columnaris probably closely resembled such recent species as Equisetum hiemale L., E. trachyodon A. Br. and others.

The stems often show a distinct swelling at the nodes; this may be due, at least in part, to the existence of transverse nodal diaphragms which enabled the dead shoots to resist contraction in the region of the nodes. The leaf-sheaths consist of numerous long and narrow segments often truncated distally, as in fig. 58, B, and as in the sheath of such a recent Horse-tail as E. ramosissimum shown in fig. 58, C. In some specimens one occasionally finds indications of delicate acuminate teeth extending above the limits of a truncated sheath. Brongniart speaks of the existence of caducous acuminate teeth in his diagnosis of the species, and the example represented in fig. 58, B, demonstrates the existence of such deciduous appendages. There is a very close resemblance between the fossil sheath of fig. 58, B, with and without the teeth, and the leaf-sheath of the recent Equisetum in fig. 58, C. In some specimens of E. columnaris in which the cast is covered with a carbonaceous film, each segment in a leaf-sheath is seen to be slightly depressed in the median portion, which is often distinctly marked by numerous small dots, the edges of the segment being flat and smooth. The median region is that in which the stomata are found and on which deposits of silica occur.

Bunbury[518] proposed the name Calamites Beani for some fossil stems from the Lower Oolite beds of the Yorkshire coast, which Bean had previously referred to in unpublished notes as C. giganteus. The latter name was not adopted by Bunbury on account of the possible confusion between this species and the Palaeozoic species Calamites gigas Brongn. The generic name Calamites must be replaced by Equisetites now that we are familiar with more perfect specimens which demonstrate the Equisetean characters of the plant.

Schimper[519] speaks of this species as possibly the pith-cast of Equisetites columnaris, but his opinion cannot be maintained; the species first described by Bunbury has considerably larger stems than those of E. columnaris. It is not impossible, however, that E. columnaris and E. Beani may be portions of the same species. The chief difference between these forms is that of size; but we have not sufficient data to justify the inclusion of both forms under one name. Zigno[520], in his work on the Oolitic Flora, figures an imperfect stem cast of E. Beani under the name of Calamites Beani, but the species has received little attention at the hands of recent writers. In 1886 Starkie Gardner[521] figured a specimen which was identified by Williamson as an example of Bunbury’s species; but the latter pointed out the greater resemblance, as regards the external appearance of the Jurassic stem, to some of the recent arborescent Gramineae[522] than to the Equisetaceae. Williamson, with his usual caution, adds that such appearances have very little taxonomic value. Fig. 60 is reproduced from the block used by Gardner in his memoir on Mesozoic Angiosperms; he quotes the specimen as possibly a Monocotyledonous stem. The fossil is an imperfect cast of a stem showing two clearly marked nodal regions, but no trace of leaf-sheaths. A recent examination of specimens in the museums of Whitby, Scarborough, York and London has convinced me that the plant named by Bunbury Calamites Beani is a large Equisetites. As a rule the specimens do not show any indications of the leaf-sheaths, but in a few cases the sheaths have left fairly distinct impressions.

In the portion of stem shown in fig. 61 the impressions of the leaf segments are clearly marked. This specimen affords much better evidence of the Equisetaceous character of the plant than those which are simply internal casts. The narrow projecting lines extending upwards from the nodes in the figured specimen probably represent the divisions between the several segments of each leaf-sheath.

In the museums of Whitby and Scarborough there are some long specimens, in one case 44 cm. in length, and 33 cm. in circumference, which are probably casts of the broad pith-cavity. These casts are often transversely broken across at the nodes, so that they consist of three or four separate pieces which fit together by clean-cut faces. This manner of occurrence is most probably due to the existence of large and resistant nodal diaphragms which separated the sand-casts of adjacent internodes. In the York museum there are some large diaphragms, 10 cm. in diameter, preserved separately in a piece of rock containing a cast of Equisetites Beani. The nodal diaphragms of some of the Carboniferous Calamites were the seat of cork development[523], and it may be that the frequent preservation of Equisetaceous diaphragms in Triassic and Jurassic rocks is due to the protection afforded by a corky investment.

The stem shown in fig. 62 appears to be a portion of a shoot of E. Beani not far from its apical region. From the lower nodes there extend clearly marked and regular lines or slight grooves tapering gradually towards the next higher node; these are no doubt the impressions of segments of leaf-sheaths. The sheaths themselves have been detached and only their impressions remain. The flattened bands at the node of the stem in fig. 60, and shown also in fig. 61, mark the place of attachment of the leaf-sheaths. On some of these nodal bands one is able to recognise small scars which are most likely the casts of outgoing leaf-trace bundles.

Some of the internal casts of this species are marked by numerous closely arranged longitudinal lines, which are probably the impressions of the inner face of a central woody cylinder. In the smaller specimen shown in fig. 62 we have the apical portion of a shoot in which the uppermost internodes are in an unexpanded condition.

It is impossible to give a satisfactory diagnosis of this species without better material. The plant is characterised chiefly by the great breadth of the stem, and by the possession of leaf-sheaths consisting of numerous long and narrow segments. Equisetites Beani must have almost equalled in size the Triassic species, E. arenaceus, described above.

This species is described at some length as affording a useful illustration of the misleading character of certain features which are entirely due to methods of preservation. The specific name was proposed by Phillips in his first edition of the Geology of the Yorkshire Coast for some very imperfect stems from the Lower Oolite rocks near Whitby[524]. The choice of the term lateralis illustrates a misconception; it was given to the plant in the belief that certain characteristic wheel-like marks on the stems were the scars of branches. Lindley and Hutton[525] figured a specimen of this species in their Fossil Flora, and quoted a remark by “Mr Williamson junior” (afterwards Prof. Williamson) that the so-called scars often occur as isolated discs in the neighbourhood of the stems. Bunbury[526] described an example of the same species with narrow spreading leaves like those of a Palaeozoic Asterophyllites, and proposed this generic name as more appropriate than Equisetites. In all probability the example shown in fig. 63 is that which Bunbury described. It is certainly the same as one figured by Zigno[527] as Calamites lateralis in his Flora fossilis formationis Oolithicae.

This specimen illustrates a further misconception in the diagnosis of the species. The long linear appendages spreading from the nodes are, I believe, slender branches and not leaves; they have not the form of delicate filmy markings on the rock face, but are comparatively thick and almost woody in appearance. The true leaves are distinctly indicated at the nodes, and exhibit the ordinary features of toothed sheaths.

Heer[528] proposed to transfer Phillips’ species to the genus Phyllotheca, and Schimper[529] preferred the generic term Schizoneura. The suggestion for the use of these two names would probably not have been made had the presence of the Equisetum sheaths been recognised.

The circular depressions a short distance above each node are the ‘branch scars’ of various writers. Schimper suggested that these radially marked circles might be displaced nodal diaphragms. Andrae[530] figured the same objects in 1853 but regarded them as branch scars, although in the specimen he describes, there are several of them lying apart from the stems, and to one of them is attached a portion of a leaf-sheath. Solms-Laubach[531] points out that the internodal position of these supposed scars is an obvious difficulty; we should not expect to find branches arising from an internode. After referring to some specimens in the Oxford museum, he adds—“In presence of these facts the usual explanation of these structures appears to me, as to Heer, very doubtful.... We are driven to the very arbitrary assumption that they represent the lowest nodes of the lateral branches which were inserted above the line of the nodes of the stem.” Circular discs similar to those of E. lateralis have been found in the Jurassic rocks of Siberia[532] and elsewhere. There are one or two examples of such discs from Siberia in the British Museum. If the nodal diaphragms were fairly hard and stout, it is easy to conceive that they might have been pressed out of their original position when the stems were flattened in the process of fossilisation. It is not quite clear what the radial spoke-like lines of the discs are due to; possibly they mark the position of bands of more resistant tissue or of outgoing strands of vascular bundles. A detached diaphragm is seen in fig. 64 C; in the centre it consists of a flat plate of tissue, and the peripheral region is traversed by the radiating lines. In the stem of fig. 64, A the deeply divided leaf-sheaths are clearly seen, and an imperfect impression of a diaphragm is preserved on the face of the middle internode. In fig. 64 B a flattened leaf-sheath is shown with the free acuminate teeth fused basally into a continuous collar[533]. The short piece of stem of Equisetites lateralis shown in fig. 58, F, shows how the free teeth may be outspread in a manner which bears some resemblance to the leaves of Phyllotheca, but a comparison with the specimens already described, and a careful examination of this specimen itself, demonstrate the generic identity of the species with Equisetites. The carbonaceous film on the surface of such stems as those of fig. 58, F, and 64, A, shows a characteristic shagreen texture which may possibly be due to the presence of silica in the epidermis as in recent Horse-tails.

There is another species of Equisetites, E. Münsteri, Schk., from a lower geological horizon which has been compared with E. lateralis, and lends support to the view that the so-called branch-scars are nodal diaphragms[534]. This species also affords additional evidence in favour of retaining the generic name Equisetites for Phillips’ species. Equisetites Münsteri is a typical Rhaetic plant; it has been found at Beyreuth and Kuhnbach, as well as in Switzerland, Hungary and elsewhere. A specimen of Equisetites originally described by Buckman as E. Brodii[535], from the Lower Lias of Worcestershire, may possibly be identical with E. Münsteri. The leaf-sheaths of this Rhaetic species consist of broad segments prolonged into acuminate teeth; some of the examples figured by Schenk[536] show clearly marked impressions of displaced nodal diaphragms exactly as in E. lateralis. Another form, Equisetum rotiferum described by Tenison-Woods[537] from Australia, is closely allied to, or possibly identical with E. lateralis.

This species of Equisetites is fairly common in the Wealden beds of the Sussex coast near Hastings, and also in Westphalia.

It is characterised by having long and slender internodes, bearing at the nodes leaf-sheaths with five or six pointed segments, and by the frequent formation of branch-tubers. These tuberous branches closely resemble those which are formed on the underground shoots of Equisetum arvense L., E. sylvaticum L. and others; they occur either singly or in chains[539]. In the specimen shown in the figure the left-hand tuber is remarkably well preserved, its surface is somewhat sunk and shrivelled, and the apex is surrounded by a nodal leaf-sheath. A thin branched root is given off just below the point of insertion of the oval tuber.

No other species of Equisetites affords such numerous examples of tubers as this Wealden plant. By some of the earlier writers the detached tubers of E. Burchardti were described as fossil seeds under the name Carpolithus.

The specimens shown in fig. 66 have been referred to another species, E. Yokoyamae Sew.[540]; they were obtained from the Wealden beds of Sussex, but according to Mr Rufford, who discovered them, the smaller tubers of this species are not found in association with those of E. Burchardti. The stems are very narrow and the tubers have a characteristic elliptical form; the species is of little value botanically, but it affords another instance of the common occurrence of these tuberous branches in the Wealden Equisetums.

Similar fossil tubers, on a much larger scale, have been found in association with the Triassic Equisetites arenaceus; with E. Parlatori Heer[541], a Tertiary species from Switzerland, and with other Mesozoic and Tertiary stems. E. Burejensis[542], described by Heer from the Jurassic rocks of Siberia, bears a close resemblance to the Wealden species.

The description of the above species by no means exhausts the material which is available towards a history of fossil Equisetums. The examples which have been selected may serve to illustrate the kind of specimens that are usually met with, as well as some of the possible sources of error which have to be borne in mind in the description of species.

Such Tertiary species as have been recorded need not be considered; they furnish us with no facts of particular interest from a morphological point of view. The wide distribution of Equisetites, especially during the Jurassic period, is one of the most interesting lessons to be learnt from a review of the fossil forms. No doubt a detailed comparison of the several species from different parts of the world would lead us to reduce the number of specific names; and at the same time it would emphasize the apparent identity of fossils which have been described from widely separated latitudes under different names.

Specimens of Equisetites are occasionally found in plant-bearing beds apart from the other members of a Flora; this isolated manner of occurrence suggests that the plant grew in a different station from that occupied by Cycads and other elements of the vegetation[543].

A selection of Triassic and Jurassic species arranged in a tabular form demonstrates the world-wide distribution of this persistent type of plant[544].

B. Phyllotheca.

The generic name Phyllotheca was proposed by Brongniart[545] in 1828 for some small fossil stems from the Hawkesbury river, near Port Jackson, Australia. The stems of this genus are divided into nodes and internodes and possess leaf-sheaths as in Equisetum, but Phyllotheca differs from other Equisetaceous plants in the form of the leaves and in the character of its sporophylls. We may define the genus as follows:—

Plants resembling in habit the recent Equisetums. Stems simple or branched, divided into distinct nodes and internodes, the latter marked by longitudinal ridges and grooves; from the nodes are given off leaf-sheaths consisting of linear-lanceolate uninerved segments coherent basally, but having the form of free narrow teeth for the greater part of their length. The long free teeth are usually spread out in the form of a cup and not adpressed to the stem, the tips of the teeth are often incurved.

The sporangia are borne on peltate sporangiophores attached to the stem between whorls of sterile leaves.

Our knowledge of Phyllotheca is unfortunately far from complete. The chief characteristic of the vegetative shoots consists in the cup-like leaf-sheaths; these are divided up into several linear segments, which differ from the teeth of an Equisetum leaf-sheath in their greater length and in their more open and spreading habit of growth. The large loose sheaths of the fertile shoots of some recent Horse-tails bear a certain resemblance to the sheaths of Phyllotheca. The diagnosis of the fertile shoots is founded principally on some Permian specimens of the genus described by Schmalhausen from Russia[546] and redescribed more recently by Solms-Laubach[547]. Prof. Zeiller[548] has, however, lately received some examples of Phyllotheca from the Coal-Measures of Asia Minor which bear strobili like those of the genus Annularia, a type which is dealt with in the succeeding chapter. A description of a few species will serve to illustrate the features usually associated with this generic type, as well as to emphasize the unsatisfactory state of our knowledge as to the real significance of such supposed generic characteristics.

There are a few fossil stems from Permian rocks of Siberia, from Jurassic strata in Italy, and from Lower Mesozoic and Permo-Carboniferous beds in South America, South Africa, India and Australia which do not conform in all points to the usually accepted definition of Equisetites, and so justify their inclusion in an allied genus. On the other hand there are numerous instances of stems or branches which have been referred to Phyllotheca on insufficient grounds. Our knowledge of this Equisetaceous plant has recently been extended by Zeiller[549], who has recorded its occurrence in the Coal-Measures of Asia Minor associated with typical Upper Carboniferous plants. The same author[550] has also brought forward good evidence for the Permian age of the beds in Siberia and Altai, where Phyllotheca has long been known. It is true that Zigno’s species of the genus occurs in Italian Jurassic rocks, but on the whole it would seem that this genus is rather a Permian than a Jurassic type. The species which Zeiller describes under the name Phyllotheca Rallii from the Coal-Measures of Herakleion (Asia Minor) shows some points of contact with Annularia. It is much to be desired, however, that we might learn more as to the reproductive organs of this member of the Equisetales; until we possess a closer acquaintance with the fructification we cannot hope to arrive at any satisfactory conclusion as to the exact position of the genus among the Calamarian and Equisetaceous forms. M. Zeiller[551] informs me that his specimens of P. Rallii, which are to be fully described in a forthcoming work, include fossil strobili resembling those of Annularia radiata. The verticils of linear leaves fused basally into a sheath agree in appearance with the star-like leaves of Annularia, but in Phyllotheca Rallii the segments appear to spread in all directions and are not extended in one plane as in the typical Annularia[552].

In an account of some fossil plants collected by Tchikatcheff in Altai, Göppert[553] describes and figures two imperfect stems of an Equisetum-like plant. Owing to the apparent absence of nodal lines on the surface of the stem the generic name Anarthrocanna is proposed for the fossils; and the manner in which the main axis appears to break up into slender branches suggested the specific name deliquescens. Schmalhausen[554] afterwards recognised the generic identity of Göppert’s fragments with the Indian and Australian stems referred to the genus Phyllotheca by McCoy[555] and Bunbury[556].

We may define the species as follows:—

Stem reaching a diameter of 2–3 cm. with internodes as much as 4 cm. long, the surface of which is traversed by longitudinal ridges and grooves which are continuous and not alternate at the nodes. Branches arise in verticils from the nodes. The leaves have the form of funnel-shaped sheaths split up into narrow and spreading linear segments, each of which is traversed by a median vein. The fertile shoot terminates in a loose strobilus bearing alternating whorls of sterile bracts and sporangiophores.

The specimens on which this diagnosis is founded are for the most part fragments of sterile branches. Some of these present the appearance of Calamitean stems in which the ridges and grooves continue in straight lines from one internode to the next. Similar stem-casts have been referred by some writers to the allied genus Schizoneura, and it would appear to be a hopeless task to decide with certainty under which generic designation such specimens should be described. The portion of stem shown in fig. 67 affords an example of an Equisetaceous plant, probably in the form of a cast of a hollow pith, which might be referred to either Phyllotheca or Schizoneura. The specimen was found in certain South African rocks which are probably of Permo-Carboniferous age[557]. It agrees closely with some stems from India described by Feistmantel[558] as Schizoneura gondwanensis, and it also resembles equally closely the Australian specimens referred by Feistmantel[559] to Phyllotheca australis and some stems of Phyllotheca indica figured by Bunbury[560].

The longitudinal ridges and grooves shown in fig. 67 probably represent the broad medullary rays and the projecting wedges of secondary wood surrounding a large hollow pith, as in Calamites. In the Calamitean casts the ridges and grooves of each internode usually alternate in position with those of the next, as in Equisetum (fig. 54, A), but in Phyllotheca, Schizoneura and Archaeocalamites there is no such regular alternation at the nodes of the internodal vascular strands.

In Phyllotheca and Schizoneura there are no casts of ‘infranodal canals’ below each nodal line, but these are by no means always found in true Calamites. It is therefore practically impossible to determine the generic position of such fossils as that shown in fig. 67 without further evidence than is afforded by leafless casts.

A few examples of Phyllotheca deliquescens have been described by Schmalhausen in which a branch bears clusters of sporangiophores, alternating with verticils of sterile bracts. The sporangiophores appear to have the form of stalked peltate appendages bearing sporangia, very similar to the sporangiophores of Equisetum. Solms-Laubach[561] has examined the best of Schmalhausen’s specimens, and a carefully drawn figure of one of the fertile branches is given in his Fossil Botany.

The significance of this manner of occurrence of sporangiophores and whorls of sterile bracts on the fertile branch will be better understood after a description of the strobilus of Calamites. In Phyllotheca the sporangiophores appear to have been given off in whorls, which were separated from one another by whorls of sterile bracts, whereas in Equisetum there are no sterile appendages associated with the sporangiophores of the strobilus, with the exception of the annulus at the base of the cone. Heer[562] first drew attention to the fact that in Phyllotheca we have a form of strobilus or fertile shoot to a certain extent intermediate in character between Equisetum and Calamites.

In abnormal fertile shoots of Equisetum, sporophylls occasionally occur above and below a sterile leaf-sheath. Potonié[563] has figured such an example in which an apical strobilus is succeeded at a lower level by a sterile leaf-sheath, and this again by a second cluster of sporophylls. As Potonié points out, this alternation of fertile and sterile members affords an interesting resemblance between Phyllotheca and Equisetum. It suggests a partial reversion towards the Calamitean type of strobilus.