Presl[914] instituted this genus for a Lower Cretaceous tree-fern from Bohemia originally figured as Lepidodendron punctatum[915] and assigned to a Palaeozoic horizon; it was afterwards named by Corda[916] Protopteris Sternbergii and referred by Brongniart[917] to Sigillaria. The genus Protopteris stands for fossil fern-stems with the habit and, in the main, the structural features of recent tree-ferns. Persistent leaf-bases and sinuous adventitious roots cover the surface of the stems: the vascular system is of the dictyostelic type characteristic of Cyathea ([fig. 240], p. 313) and Alsophila. It is by the pattern formed by the vascular tissue on the exposed surface of the leaf-bases that Protopteris is most readily recognised: the leaf-trace has a horse-shoe form with the ends curled inwards and the sides more or less indented ([fig. 277]). The generic name Caulopteris is used by some authors in preference to Presl’s genus; but Protopteris is more conveniently restricted to Mesozoic Cyatheaceous stems and Caulopteris to Palaeozoic stems, with the internal structure of Psaronius (see Chap. XXIII.). Stenzel applies Caulopteris to Mesozoic stems in which the leaf-trace consists of several separate strands and not of a continuous band.
Fig. 274. Coniopteris arguta. (Nat. size. From a specimen in the Sedgwick Museum, Cambridge.)
Fig. 275.
- Coniopteris arguta. (Fertile pinnae; nat. size.)
- C. hymenophylloides.
A, from the Inferior Oolite of Yorkshire (British Museum); B, from Jurassic rocks in Turkestan.
Lower Cretaceous casts of tree-fern stems in the Prague Museum have been described under the names Alsophilina and Oncopteris; the figures of the latter ([fig. 276]) given by Feistmantel[918] and by Velenovský[919] show the petiole-bases arranged in vertical rows and characterised by leaf-traces consisting of two separate strands in the form of two Vs lying on their sides.
Tree-fern stems described under various generic names are not infrequently found in European Lower Cretaceous rocks: their comparative abundance affords an example of striking changes in geographical distribution since the latter part of the Mesozoic epoch. The Cyatheaceae no longer exist in Europe and the arborescent species of the genus have retreated to more southern regions.
