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.
Fig. 67. Phyllotheca? ¾ nat. size. From a South African specimen of Permo-Carboniferous age in the British Museum.
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.
2. Phyllotheca Brongniarti Zigno. Fig. 68, A.
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.