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.