The genus Schizoneura is too imperfectly known to be defined with any approach to completeness, or to be assigned to a family defined within certain prescribed limits. Phyllotheca is another genus about which we possess but little satisfactory knowledge; we are still without evidence as to its structure, and the descriptions of the few strobili that are known are not consistent. Recent work points to a probability of Phyllotheca being closely allied to Annularia, a genus included in the Calamarieae, and standing for a certain type of Calamitean foliage-shoots.
In comparing the Calamarieae with the Equisetaceae, the alternation of sterile and fertile whorls in the strobilus, and the free linear leaves at the nodes instead of leaf-sheaths are two characters made use of as distinguishing features of the genus Calamites as the type of the Calamarieae. On the other hand, the strobili of Phyllotheca appear to agree with those of Calamites rather than with those of Equisetum, and strobili of Archaeocalamites have been found exhibiting the typical Equisetum characters. The sheath-like form of the leaves is not necessarily peculiar to the Equisetaceae, and we have evidence that leaf-sheaths occurred on the nodes of Calamitean plants. In Archaeocalamites the leaves possess characteristic features, and can hardly be said to agree more closely with those of Calamites than with the leaves of Phyllotheca or Sphenophyllum, a genus belonging to another class of Pteridophytes.
On the whole, then, without discussing further the possibilities of a subdivision of the Equisetales, we may regard the genera Calamites, Archaeocalamites, Equisetites, Equisetum, Phyllotheca and Schizoneura as so many members of the Equisetales, without insisting on a classification which cannot be supported by satisfactory evidence.
Our knowledge of Calamites is fairly complete. Abundant and well-preserved material from the Coal-Measures of England, and from Permo-Carboniferous rocks of France, Germany and elsewhere, has enabled palaeobotanists to investigate the anatomical characters of both the vegetative and reproductive structures of this genus. We are in a position to give a detailed diagnosis of Calamitean stems, roots and strobili, and to determine the place of this type of plant in a system of classification. Calamites not only illustrates the possibilities of palaeobotanical research, but it demonstrates the importance of fossil forms as foundations on which to construct the most rational classification of existing plants. The close alliance between Calamites and the recent Equisetaceae has been clearly established, and certain characteristics of the former genus render necessary an extension and modification of the definition of the class to which both Calamites and Equisetites belong. The Calamites broaden our conception of the Equisetaceous alliance, and by their resemblance to other extinct Palaeozoic types they furnish us with important links towards a phylogenetic series, which the other members of the Equisetales do not supply.
From the Upper Devonian to the Permian epoch Calamites and other closely related types played a prominent part in the vegetation of the world. We have no good evidence for the existence of Calamites in Triassic times; in its place there were gigantic Equisetums which resembled modern Horse-tails in a remarkable degree. In the succeeding Jurassic period tree-like Equisetums were still in existence, and species of Equisetites are met with in rocks of this age in nearly all parts of the world. A few widely distributed species are known from Wealden rocks, but as we ascend the geologic series from the Jurassic strata, the Equisetums become less numerous and the individual plants gradually assume proportions practically identical with those of existing forms.
A. Equisetites.
The generic name Equisetites was proposed by Sternberg in 1838[496] as a convenient designation for fossil stems bearing a close resemblance to recent species of Equisetum. Some authors have preferred to apply the name Equisetum to fossil and recent species alike, but in spite of the apparent identity in the external characters of the fossil stems with those of existing Horse-tails, and a close similarity as regards the cones, there are certain reasons for retaining Sternberg’s generic name. It is important to avoid such nomenclature as might appear to express more than the facts admit. If the custom of adding the termination -ites to the root of a recent generic term is generally followed, it at once serves to show that the plants so named are fossil and not recent species. Moreover, in the case of fossil Equisetums we know nothing of their internal structure, and our comparisons are limited to external characters. Stems, cones, tubers, and leaves are often very well preserved as sandstone casts with distinct surface-markings, but we are still in want of petrified specimens. There is indeed evidence that some of the Triassic and Jurassic species of Equisetites, like the older Calamites, possessed the power of secondary growth in thickness, but our deductions are based solely on external characters.
In the following pages a few of the better known species of Equisetites are briefly described, the examples being chosen partly with a view to illustrate the geological history of the genus, and partly to contribute something towards a fuller knowledge of particular species. One of the most striking facts to be gleaned from a general survey of the past history of the Equisetaceae is the persistence since the latter part of the Palaeozoic period of that type of plant which is represented by existing Equisetums. There is perhaps no genus in existence which illustrates more vividly than Equisetum the survival of an extremely ancient group, which is represented to-day by numerous and widely spread species. The Equisetaceous characteristics mark an isolated division of existing Vascular Cryptogams, and without reference to extinct types it is practically impossible to do more than vaguely guess at the genealogical connections of the family. When we go back to Palaeozoic plants there are indications of guiding lines which point the way to connecting branches between the older Equisetales and other classes of Pteridophytes. The recently discovered genus Cheirostrobus[497] is especially important from this point of view.
LEAF-SHEATHS OF EQUISETITES.
The accurate description of species, and the determination of the value of such differences as are exhibited in the surface characters of structureless casts, are practically impossible in many of the fossil forms. In certain living Horse-tails we find striking differences between fertile and sterile shoots, and between branches of different orders. The isolated occurrence of fragments of fossil stems often leads to an artificial separation of ‘species’ largely founded on differences in diameter, or on slight variations in the form of the leaf-sheaths. It is wiser to admit that in many cases we are without the means of accurate diagnosis, and that the specific names applied to fossil Equisetums do not always possess much value as criteria of taxonomic differences.