Dichopteris.
This genus was proposed by Zigno[1469] for some large specimens from the Jurassic plant-beds of Northern Italy.
The bipinnate leaves are characterised by the great breadth of the rachis which is dichotomously branched in the distal region ([fig. 363]); the linear pinnae reach a considerable length. Pinnules relatively small, oblong and slightly contracted at the base; the decurrent and confluent lamina forms a narrow wing to the main axis. Veins slightly divergent and forked, as in Ptilozamites.
Dichopteris visianica, Zigno. Fig. 363.
A specimen of this species in the Padua Museum has a total length of 83 cm. It has been elsewhere suggested[1470] that a fragment figured by Zigno as a fertile example of this type is probably part of a frond of the Osmundaceous fern Todites. Since this opinion was expressed I have had an opportunity of examining the actual specimen at Padua: the circular patches described by Zigno as sori appear to be irregularities in the matrix and not an original feature.
Brongniart[1471] instituted the genus Pachypteris for some imperfectly preserved English Jurassic fossils from Whitby, which he described as P. lanceolata. Specimens have since been described[1472] from the Inferior Oolite rocks of the Yorkshire coast. Brongniart described the pinnules as being without veins or as possessing only a midrib. It is almost certain that the apparent absence of veins in most specimens[1473] is due to the fleshy nature of the segments and that the species P. lanceolata should be transferred to Dichopteris.
Krasser[1474] has described a species from Cretaceous rocks of the island of Lesina, off the Dalmatian coast, as Pachypteris dalmatica which is very similar in habit to the English specimens and to Zigno’s Dichopteris visianica. One of Krasser’s specimens is practically identical with Dichopteris lanceolata (Brongn.), while in others the small pinnules are replaced in some of the pinnae by a continuous lamina with a few distal serrations. The latter form a link between the Dichopteris and Thinnfeldia type of segment. Krasser gives a full résumé of opinions expressed by other authors in regard to the position of Pachypteris (= Dichopteris) and decides in favour of a Cycadean alliance.
Fig. 363. Dichopteris visianica, Zigno. (⅓ nat. size. After Zigno.)
A French Jurassic plant which Saporta[1475] made the type of a new genus Scleropteris, and described as S. Pomelii, appears to be indistinguishable from Dichopteris.
Dichopteris, though conveniently retained as a distinct genus, agrees so closely, in the broad and forked rachis and in the fleshy pinnules, with Thinnfeldia that it would seem reasonable to regard the two genera as members of the same group.
Several authors have drawn attention to the striking resemblance in form and venation between the fronds of the Palaeozoic genus Odontopteris and those of Ctenopteris and Thinnfeldia. In Odontopteris, as in Neuropteris, another Palaeozoic genus, the rachis occasionally bifurcates as in Thinnfeldia and Dichopteris, and the ultimate segments of some species of Odontopteris ([fig. 366], A) are practically identical with those of Thinnfeldia and Ptilozamites.
Odontopteris is probably a Pteridosperm. There is no adequate reason for supposing that this group of plants which played a prominent part in the Permo-Carboniferous floras was no longer in existence during the Mesozoic era.