Rhaetic leaves of the type represented by S. rhoifolia have a wide geographical distribution.
The specimens described by Feistmantel from the Damuda series of India as Sagenopteris longifolia are no doubt fronds of Glossopteris longifolia[1250].
The Wealden species Sagenopteris Mantelli (Dunk.)[1251] agrees closely in habit and in the form of the leaflets with S. Phillipsi and S. rhoifolia. It is probable that some of the leaves described by Velenovský[1252] from Lower Cretaceous rocks in Bohemia as Thinnfeldia variabilis are portions of Sagenopteris fronds. S. Mantelli is recorded from several European localities, from California[1253], and elsewhere.
Sagenopteris appears to have been widely distributed during the Rhaetic, Jurassic and Lower Cretaceous floras. The very great similarity between the specimens recorded from these three formations renders the genus an uncertain guide in regard to geological age. Decisive evidence as to its position in the plant kingdom is at present lacking: the inclusion of the genus as a possible member of the Hydropterideae has still to be justified.
CHAPTER XXVII.
GENERA OF PTERIDOSPERMS, FERNS, AND PLANTAE INCERTAE SEDIS.
The genera and species described in this Chapter are founded on sterile leaves or portions of leaves, and in the great majority of cases the reproductive organs are either imperfectly known or have still to be discovered. Some of the genera, the smaller number, are no doubt true ferns, while most of them may safely be regarded as plants which will ultimately be shown to belong to some other group, in most cases that of the Pteridosperms. It is possible that a few of the types may be members of the Cycadophyta rather than of the Pteridospermeae, but evidence as to systematic position is for the most part of a negative kind or too incomplete to lead to any definite expression of opinion as to the cycadean or pteridosperm nature of the imperfectly known Palaeozoic or Mesozoic species. Many of the genera are of little botanical interest, though even the most problematical are of importance as criteria of geological age. Genera which there is good reason for including in the Pteridosperms are dealt with in this section, in order that the Chapter in Volume III. devoted to this important group may be limited to more completely known types.
In most text-books it is customary to employ family names for sterile fern-like fronds which possess similar venation features or have in common certain vegetative characters, the value of which it is impossible to estimate. In the following account family or group names are not adopted, on the ground that such slight utility as they may have is more than counterbalanced by the risk attending a grouping under one name of plants which may agree only in unessential characters. The practice of classifying fossil plants has been carried to excess. Grouping together genera as a matter of convenience unavoidably creates a prejudice in favour of actual relationship, which may or may not exist.