Heer[528] proposed to transfer Phillips’ species to the genus Phyllotheca, and Schimper[529] preferred the generic term Schizoneura. The suggestion for the use of these two names would probably not have been made had the presence of the Equisetum sheaths been recognised.
The circular depressions a short distance above each node are the ‘branch scars’ of various writers. Schimper suggested that these radially marked circles might be displaced nodal diaphragms. Andrae[530] figured the same objects in 1853 but regarded them as branch scars, although in the specimen he describes, there are several of them lying apart from the stems, and to one of them is attached a portion of a leaf-sheath. Solms-Laubach[531] points out that the internodal position of these supposed scars is an obvious difficulty; we should not expect to find branches arising from an internode. After referring to some specimens in the Oxford museum, he adds—“In presence of these facts the usual explanation of these structures appears to me, as to Heer, very doubtful.... We are driven to the very arbitrary assumption that they represent the lowest nodes of the lateral branches which were inserted above the line of the nodes of the stem.” Circular discs similar to those of E. lateralis have been found in the Jurassic rocks of Siberia[532] and elsewhere. There are one or two examples of such discs from Siberia in the British Museum. If the nodal diaphragms were fairly hard and stout, it is easy to conceive that they might have been pressed out of their original position when the stems were flattened in the process of fossilisation. It is not quite clear what the radial spoke-like lines of the discs are due to; possibly they mark the position of bands of more resistant tissue or of outgoing strands of vascular bundles. A detached diaphragm is seen in fig. 64 C; in the centre it consists of a flat plate of tissue, and the peripheral region is traversed by the radiating lines. In the stem of fig. 64, A the deeply divided leaf-sheaths are clearly seen, and an imperfect impression of a diaphragm is preserved on the face of the middle internode. In fig. 64 B a flattened leaf-sheath is shown with the free acuminate teeth fused basally into a continuous collar[533]. The short piece of stem of Equisetites lateralis shown in fig. 58, F, shows how the free teeth may be outspread in a manner which bears some resemblance to the leaves of Phyllotheca, but a comparison with the specimens already described, and a careful examination of this specimen itself, demonstrate the generic identity of the species with Equisetites. The carbonaceous film on the surface of such stems as those of fig. 58, F, and 64, A, shows a characteristic shagreen texture which may possibly be due to the presence of silica in the epidermis as in recent Horse-tails.
There is another species of Equisetites, E. Münsteri, Schk., from a lower geological horizon which has been compared with E. lateralis, and lends support to the view that the so-called branch-scars are nodal diaphragms[534]. This species also affords additional evidence in favour of retaining the generic name Equisetites for Phillips’ species. Equisetites Münsteri is a typical Rhaetic plant; it has been found at Beyreuth and Kuhnbach, as well as in Switzerland, Hungary and elsewhere. A specimen of Equisetites originally described by Buckman as E. Brodii[535], from the Lower Lias of Worcestershire, may possibly be identical with E. Münsteri. The leaf-sheaths of this Rhaetic species consist of broad segments prolonged into acuminate teeth; some of the examples figured by Schenk[536] show clearly marked impressions of displaced nodal diaphragms exactly as in E. lateralis. Another form, Equisetum rotiferum described by Tenison-Woods[537] from Australia, is closely allied to, or possibly identical with E. lateralis.
Fig. 64. Equisetites lateralis Phill. A. Part of a stem showing leaf-sheaths and an imperfect diaphragm. B. A single flattened leaf-sheath. C. A detached nodal diaphragm. From a specimen in the York Museum. Slightly reduced.
8. Equisetites Burchardti Dunker[538]. Fig. 65.
This species of Equisetites is fairly common in the Wealden beds of the Sussex coast near Hastings, and also in Westphalia.
Fig. 65. Equisetites Burchardti Dunk. Showing a node with two tubers and a root. From a specimen in the British Museum. Nat. size.
It is characterised by having long and slender internodes, bearing at the nodes leaf-sheaths with five or six pointed segments, and by the frequent formation of branch-tubers. These tuberous branches closely resemble those which are formed on the underground shoots of Equisetum arvense L., E. sylvaticum L. and others; they occur either singly or in chains[539]. In the specimen shown in the figure the left-hand tuber is remarkably well preserved, its surface is somewhat sunk and shrivelled, and the apex is surrounded by a nodal leaf-sheath. A thin branched root is given off just below the point of insertion of the oval tuber.