This species of Phyllotheca from the Lower Oolite rocks of Italy is known only in the form of sterile branches. The leaves are fused basally into an open cup-like sheath which is dissected into several spreading and incurved linear segments. The internodes are striated longitudinally; they are about 2 mm. in diameter and 10 mm. in length.
The specimen represented in fig. 68, A, was originally described by the Italian palaeobotanist Zigno[564]; it serves to illustrate the points of difference between this genus and the ordinary Equisetum. The open and spreading sheaths clasping the nodes and the erect solitary branches give the plant a distinctive appearance.
- Phyllotheca Brongniarti, Zigno. Nat. size. (After Zigno.)
- Calamocladus frondosus, Grand’Eury. (After Grand’Eury.) Slightly enlarged.
- Phyllotheca indica, Bunb. Part of a leaf-sheath. From a specimen in the Museum of the Geological Society. Slightly enlarged.
Sir Charles Bunbury[565] described several imperfect specimens from the Nagpur district of India under this name, but he expressed the opinion that it was not clear to him if the plant was specifically distinct from the Phyllotheca australis Brongn. previously recorded from New South Wales. Feistmantel[566] subsequently described a few other Indian specimens, but did not materially add to our knowledge of the genus. Bunbury’s specimens were obtained from Bharatwádá in Nagpur, in beds belonging to the Damuda series of the Lower Gondwana rocks, usually regarded as of about the same age as the Permian rocks of Europe.
Phyllotheca indica is represented by broken and imperfect fragments of leaf-bearing stems. The species is thus diagnosed by Bunbury:—“Stem branched, furrowed; sheaths lax, somewhat bell-shaped, distinctly striated; leaves narrow linear, with a strong and distinct midrib, widely spreading and often recurved, nearly twice as long as the sheaths.” An examination of the specimens in the Museum of the Geological Society of London, on which this account was based, has led me to the opinion that it is practically impossible to distinguish the Indian examples from P. australis described by Brongniart[567] from New South Wales. The few specimens of the latter species which I have had an opportunity of examining bear out this view. In the smaller branches the axis of P. indica is divided into rather short internodes on which the ridges and grooves are faintly marked. In the larger stems the ridges and grooves are much more prominent, and continuous in direction from one internode to the next; a few branches are given off from the nodes of some of the specimens. The leaves are not very well preserved; they consist of a narrow collar-like basal sheath divided up into numerous, long and narrow segments, which are several times as long as the breadth of the sheath, and not merely twice as long as Bunbury described them. Each leaf-sheath has the form of a very shallow cup-like rim clasping the stem at a node, with long free spreading segments which are often bent back in their distal region. The general habit of the leafy branches appears to be identical with that of P. australis as figured by McCoy.
Prof. Zeiller informs me that in the type-specimen on which Brongniart founded the species, P. australis, the sheath appears to be closely applied to the stem with a verticil of narrow spreading segments radiating from its margin. It may be, therefore, that in the Australian form there was not such an open and cup-like sheath as in P. indica; but it would be difficult, without better material before us, to feel confidence in any well marked specific distinctions between the Indian and Australian Phyllothecas.
On the broader stems, such as that of fig. 67, we have clearly marked narrow grooves and broader and slightly convex ridges, which present an appearance identical with that of some Calamitean stems. In the specimen figured by Bunbury[568] in his Pl. X, fig. 6, there is a circular depression on the line of the node which represents the impression of the basal end of a branch; on the edges of the node there are indications of two other lateral branches. The nature of this stem-cast points umnistakeably to a woody stem like that of Calamites. The precise meaning of the ridges and grooves on the cast is described in the Chapter dealing with Calamitean plants.
Grand’Eury[569] in his monograph on the coal-basin of Gard, has recently described under the name of Calamocladus frondosus what he believes to be the leaf-bearing axes of a Calamitean plant. The thicker branches are almost exactly identical in appearance with the broader specimens of Phyllotheca. The finer branches of Calamocladus bear cup-like leaf-sheaths which are divided into long and narrow recurved segments (fig. 67, B), precisely as in Phyllotheca. These comparisons lead one to the opinion that the Phyllotheca of Australia and India may be a close ally of the Permo-Carboniferous Calamitean plants. The form of the leaf-whorls of Annularia (Calamarian leaf-bearing branches) and of Calamocladus is of the same type as in Phyllotheca; the character of the medullary casts is also the same. The nature of the fertile shoot of Phyllotheca described by Schmalhausen from Siberia, with its alternating whorls of sterile and fertile leaves, is another point of agreement between this genus and Calamitean plants. An Equisetaceous species has been described from the Newcastle Coal-Measures of Australia by Etheridge[570] in which there are two forms of leaves, some of which closely resemble those of Phyllotheca indica, while others are compared with the sterile bracts of Cingularia, a Calamitean genus instituted by Weiss[571].
When we turn to other recorded forms of Phyllotheca many of them appear on examination to have been placed in this genus on unsatisfactory grounds. Heer figures several stem fragments from the Jurassic rocks of Siberia as P. Sibirica Heer[572], and it was the resemblance between this form and the English Equisetites lateralis which led to the substitution of Phyllotheca for Equisetites in the latter species. Without examining Heer’s material it is impossible to criticise his conclusions with any completeness, but several of his specimens, appear to possess leaf-sheaths more like those of Equisetum than of Phyllotheca.
The frequent occurrence of isolated diaphragms and the comparatively long acuminate teeth of the leaf-sheath afford obvious points of resemblance to Equisetites lateralis. Some of the examples figured by Heer appear to be stem fragments, with numerous long and narrow filiform leaves different in appearance from those of other specimens which he figures. It may be that some of the less distinct pieces of stems are badly torn specimens in which the internodes have been divided into filiform threads. Heer also figures a fertile axis associated with the sterile stems, and this does not, as Heer admits, show the alternating sterile bracts such as Schmalhausen has described. So far as it is possible to judge from an examination of Heer’s figures and a few specimens from Siberia in the British Museum—and this is by no means a safe basis on which to found definite opinions—there appears to be little evidence in favour of separating the fossils described as Phyllotheca Sibirica from Equisetites. This Siberian form may indeed be specifically identical with Equisetites lateralis Phill.
Various species of Phyllotheca have been described from Jurassic and Upper Palaeozoic rocks in Australia. Some of these possess cup-like leaf-sheaths, and in the case of the thicker specimens they show continuous ridges and grooves on the internodes, as well as a habit of branching similar to that in some of the Italian Phyllothecas. In some of the stems it is however difficult to recognise any characters which justify the use of the term Phyllotheca. A fragment figured by Tenison-Woods[573] as a new species of Phyllotheca, P. carnosa, from Ipswich, Queensland, affords an example of the worthless material on which species have not infrequently been founded. The author of the species describes his single specimen as a “faint impression”; the figure accompanying his description suggests a fragment of some coniferous branch, as Feistmantel has pointed out in his monograph on Australian plants.
It is important that a thorough comparative examination should be made of the various fossil Phyllothecas with a view to determine their scientific value, and to discover how far the separation of Phyllotheca and Equisetites is legitimate in each case. There is too often a tendency to allow geographical distribution to decide the adoption of a particular generic name, and this seems to have been especially the case as regards several Mesozoic and Palaeozoic Southern Hemisphere plants.
The geological and geographical range of Phyllotheca is a question of considerable interest, but as already pointed out it is desirable to carefully examine the various records of the genus before attempting to generalise as to the range of the species. Phyllotheca is often spoken of as a characteristic member of the Glossopteris Flora of the Southern Hemisphere, and its geological age is usually considered to be Mesozoic rather than Palaeozoic.
C. Schizoneura.
The plants included under this genus were originally designated by Brongniart[574] Convallarites and classed as Monocotyledons. Some years later Schimper and Mougeot[575] had the opportunity of examining more perfect material from the Bunter beds of the Vosges, and proposed the new name Schizoneura in place of Brongniart’s term, on the grounds that the specimens were in all probability portions of Equisetaceous stems, and not Monocotyledons. Our knowledge of this genus is very limited, but the characteristics are on the whole better defined than in the case of Phyllotheca. The following diagnosis illustrates the chief features of Schizoneura.
Hollow stems with nodes and internodes as in Equisetum; the surface of the internodes is traversed by regular ridges and grooves, which are continuous and not alternate in their course from one internode to the next. The leaf-sheaths are large and consist of several coherent segments; the sheaths are usually split into two or more elongate ovate lobes, and each lobe contains more than one vein. Fertile shoots are unknown.
Two of the best known and most satisfactory species are Schizoneura gondwanensis Feist. and S. paradoxa Schimp. and Moug.
This species is represented by numerous specimens from the Lower Gondwana rocks of India[576]; it is characterised by narrow articulated stems which bear large leaf-sheaths at the nodes. The sheaths may have the form of two large and spreading elongate-oval lobes, each of which is traversed by several veins (fig. 69, B), or the lobes may be further dissected into long linear single-veined segments, as in fig. 69, A. It is supposed that in the young condition each node bears a leaf-sheath consisting of laterally coherent segments which, as development proceeds, split into two or more lobes. Feistmantel records this species from the Talchir, Damuda and Panchet divisions of the Lower Gondwana series of India; these divisions are regarded as equivalent to the Permo-Carboniferous and Triassic rocks of Europe. The two specimens shown in fig. 69 are from the Lower Gondwana rocks of the Raniganj Coal-field, India.
As already pointed out[577], some of the specimens of flat and broader stems referred by Feistmantel to Schizoneura are identical in appearance with stems which have been described from India and elsewhere as species of Phyllotheca.
There are a few specimens of S. gondwanensis in the British Museum, but the genus is poorly represented in European collections.
A similar plant was described in 1844 by Schimper and Mougeot[578] from the Bunter rocks of the Vosges as Schizoneura paradoxa. This species bears a very close resemblance to the Indian forms, and indeed it is difficult to point to any distinction of taxonomic importance. Feistmantel considers that the European plant has rather fewer segments in the leaf-sheaths, and that the Indian plant had somewhat stronger stems. Both of these differences are such as might easily be found on branches of the same species. It is, however, interesting to notice the very close resemblance between the Lower Trias European plant and the somewhat older member of the Glossopteris flora recorded from India and other regions, which probably once formed part of that Southern Hemisphere Continent which is known as Gondwana Land[579].