. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 372 EOUtSETALES separated by leaf-sheaths (Fig. 196). It is thus seen that the strobilus of Equisetum is not always that circumscribed terminal body which is typical for the living species. The Equisetum-type has been recognised, though with some uncertainty, and only in few specimens, as far back as the Middle Coal Measures;' but it is seen represented more commonly, and by large forms, in the Mesozoic rocks. Related to it are two other fossil forms: the genus Phyllotheca of Permian age resembles Equ
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 372 EOUtSETALES separated by leaf-sheaths (Fig. 196). It is thus seen that the strobilus of Equisetum is not always that circumscribed terminal body which is typical for the living species. The Equisetum-type has been recognised, though with some uncertainty, and only in few specimens, as far back as the Middle Coal Measures;' but it is seen represented more commonly, and by large forms, in the Mesozoic rocks. Related to it are two other fossil forms: the genus Phyllotheca of Permian age resembles Equisetum in the general features of the shoot, with its cup-like leaf-sheaths webbed at the base, but differing in the form of the leaves and in the fertile region : this is constructed on the general plan of Mquisetum, but with the strobilus interrupted at intervals by sheaths of sterile leaves, as in some abnormal con- ditions of Equisetum (Fig. 197). Some specimens of Phyllotheca have, however, been described by M. Zeiller as having strobili like those of Aunularia, that is, of the Calamostachys-type? The other genus is Schizoneura, of Triassic age, characterised by the whorled leaves being associated in webbed sheaths, which may, however, be slit longitudinally to the base. They thus form leaf-like lobes which stand off at a considerable angle from the axis (Fig. 198). The axis is marked by longi- tudinal grooves, which are continuous longitudinally from internode to internode, thus showing that the leaves of successive whorls did not alternate. The fructifi- cation is unknown. Most of the older Equisetal fossils, however, belong to the Calamarian type. These plants were often of dendroid habit, the stem, but with a similar primary construction of the shoot to that seen in Equisetum. The leaf-whorls are frequently webbed at the base, though often only slightly, as in Annularia; but in Asterophyllites, which is traced back to the Devonian period, the leaves appear quite separa
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