Corda[1132] proposed the generic name _Zygopteris_ for petrified petioles from the Permian of Saxony, included by Cotta in his genus _Tubicaulis_, which he named _T. primarius_. Corda’s genus has been generally used for petioles of Palaeozoic ferns characterised by a vascular strand having the form of an =H= in transverse section (fig. 308, D). Since the generic name was instituted, information has been obtained in regard to the nature of the stems which bore some of the petioles of the _Zygopteris_ type; and for other species of _Zygopteris_, the stems of which are still unknown, new generic names have been proposed. P. Bertrand[1133] retains _Zygopteris_ for one species only, _Z. primaria_. Fig. 308, D, shows the character of the petiolar vascular strand; the chief points are the comparatively long cross-pieces (antennae of P. Bertrand) inclined at an angle of 45° to the plane of symmetry of the petiole axis, and the groups of protoxylem elements shown by the white patches in fig. D. In this as in other members of the Zygoptereae the main rachis of the leaf gives off four sets of branches in pairs alternately from the right and left side of the primary vascular axis. This method of branching of the stele in the primary rachis of several members of the Coenopterideae shows that the fronds bore pinnae laterally disposed, in some cases in one row and in others in two rows on each side of the rachis. In a typical fern frond, as represented by recent and most fossil species, branching of the rachis occurs in the plane of the frond, that is in the plane represented by the horizontal arm of xylem in _Zygopteris primaria_ connecting the two antennae or cross-pieces. In the Zygoptereae the branches from the petiole vascular axis lie in a plane at right angles to that of the frond; they lie in the transverse and not in the horizontal plane. The two strands shown in fig. 308, B, 4, have been formed by the division of a single strand, 3, in the transverse plane (i.e. in the plane of the paper). As Tansley[1134] points out, a type of branching superficially similar to, though not identical with this, is seen in some recent species of _Gleichenia_ and _Lygodium_. In this connexion it is worthy of note that a fern figured by Unger from Thuringia as _Sphenopteris petiolata_ Goepp[1135] bears pinnae in two rows on the rachis which are characterised by repeated branching and by a very narrow lamina or by slender naked axes; the occurrence of this form of frond in rocks containing _Clepsydropsis antiqua_ (fig. 308, A) suggests a possible connexion between the petrified rachis and the impressions of the leaves. [Illustration: FIG. 308. A. _Clepsydropsis antiqua._ B. _Etapteris Scotti._ C. _Diplolabis forensis._ D. _Zygopteris primaria._ E–G. _Stauropteris oldhamia._ The white patches in the xylem in figs. B–G mark the position of protoxylem elements. (A, after Unger; B–G, after P. Bertrand.)] [Illustration: FIG. 309.