Dictyophyllum is represented by several types to which various specific names have been assigned, the distinguishing features being the form of the pinna lobes, the degree of concrescence between the basal portions of the pinnae, and similar features which in some cases can only be safely used as criteria when large specimens are available for comparison.

Dictyophyllum exile (Brauns). Figs. [281], [282], D, E.
1862.Camptopteris exilis, Brauns, Palaeontograph. IX. p. 54.
1867.Dictyophyllum acutilobum, Schenk, Foss. Flor. Grenz. p. 77, Pls. XIX. XX.
1878.D. exile, Nathorst, Flora vid Bjuf, I. p. 39, Pl. V. fig. 7.
—D. acutilobum, ibid. Pl. XI. fig. 1.

The restoration, after Nathorst[947], shown in [fig. 281] illustrates the habit of this striking fern, examples of which or of closely allied species are recorded from Rhaetic rocks of Germany, Scania, Persia, Bornholm, Tonkin, China, and elsewhere[948]. The petiole, reaching a length of 60 cm., forks at the apex into two equal arms leaving between them an oval space and occasionally crossing one another. The axes of these branches are twisted so that the pinnae, which may be as many as 24 on each arm, and arise from the inner side, by torsion of the axes assume an external position. An interesting analogy as regards the twisted rachis of Dictyophyllum exile and Camptopteris is afforded by the leaves of the Cycads, Macrozamia Fawcettiae and M. corallipes, which are also characterised by the torsion of the rachis. The habit, justly compared by Nathorst with that of Matonia pectinata, affords another illustration of the common occurrence in older ferns of a dichotomous system of branching. The pinnae, characterised by circinate vernation, reach a length of 60 cm. and are divided into linear lobes inclined obliquely or at right angles to the pinna axis. The whole of the under surface of the lamina may be covered with sporangia, 4–7 sporangia in each sorus; the annulus is incomplete and approximately vertical ([fig. 282], D). The rhizome is probably represented by the dichotomously branched axis described by Nathorst from Scania as Rhizomopteris major; the leaf-scars show a horse-shoe leaf-trace.

Fig. 282.

(After Nathorst; A, B, C, E, ⅔ nat. size.)

Dictyophyllum Nathorsti Zeiller[949].

This type, represented by a splendid series of specimens from the Rhaetic beds of Tonkin, agrees very closely with D. exile. It differs, however, in the basal parts of the pinnae which are concrescent for a length of 5 to 8 cm. instead of free as in D. exile; and, to a slight degree, in the form of the ultimate segments. In habit and in soral characters the two species are practically identical. Each sorus contains 5 to 8 sporangia, which are rather larger than those of Dipteris.