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 im- 

 perfectly known Palaeozoic or Mesozoic species. Many of the 

 genera are of little botanical ititerest, though even the most prob- 

 lematical 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 counter- 



