38 



have at their disposal to-day, as compared to that which was 

 accessible to earlier workers. In the writings of these authors 

 many of the provisional and artificial terms are replaced by new 

 generic names of real taxonomic value, and resting on such a basis 

 as would be recognized as reasonable by modern botanists. 



In working through the specimens of ferns mentioned in the 

 present Catalogue the difficulty of coming to satisfactory conclu- 

 sions as to limits of specific distinctions, and the claim of isolated 

 fragments to be included in one or other genus or species, has been 

 demonstrated with only too much clearness. It is impossible to 

 deny that many of our fossil species, determined by those much 

 more fully equipped with the requisite knowledge and experience 

 than myself, are of very doubtful value. Granting the truth of 

 this assertion, it must be remembered that specific distinctions and 

 definitions are of very great importance in fossil botany, and if we 

 waited until our hands were full of all the needful data we should 

 never make any appreciable advance in the analysis of geologic 

 floras. We must recognize that the numerous species of fossil 

 plants do not all rest on unequivocal foundations ; but they are at 

 all events not quite so meaningless and unscientific as some would 

 lead us to believe. 



The Wealden material affords many interesting examples of the 

 importance of bearing in mind the great differences in species 

 which are solely due to the manner of preservation, and the 

 character of the matrix in which the fossils are found. Some of 

 the fragments, marked out in clear dark-brown lines on a fine 

 homogeneous stone, present a very different appearance when seen 

 as carbonaceous impressions on the surface of a grey-coloured clay. 

 Again, such characters as the shape of the pinnules, whether 

 acutely terminated or with obtuse apices, whether with entire 

 margins or slightly dentate, are to be used with extreme care in 

 view of the different forms which appear in the ultimate segments 

 of recent fronds, according as they are fresh and flat, or dry and 

 more or less curled over at the edges. The position of the pinnaB 

 on the main rachis, whether alternate or opposite, is of little value 

 in the case of fragmentary remains ; in recent fronds the pinnae 

 may be opposite towards one end of the rachis, and distinctly 

 alternate towards the other. Another character which I am 

 disposed to regard as of little value is the relative size of the 

 fronds, and also the size of the ultimate divisions. Probably the 



