XVI FOSSIL ARCHEGONIATES 579 



Filicinecu (Potonie (j); Scott (/)) 



The great majority of the fossil remains of Ferns are in the 

 forms of impressions, but these are frequently of great clear- 

 ness, the numerous Carboniferous fossils being especially beau- 

 tiful, and showing all the external characters most perfectly. 

 As these impressions are usually of sterile leaves, the first at- 

 tempts to classify them were based upon the venation. While 

 the venation is a diagnostic character of importance, it cannot 

 be relied upon exclusively, as it sometimes happens that two 

 nearly related forms, c. g., Onoclca scnsibilis and O. striithi- 

 opteris, have a very different type of venation. On the other 

 hand, the Cycad, Stangcria, has a venation so much like that of 

 a Fern that the sterile plant was at first described as a species 



of Lomaria. 



The more recent students of fossil plant remains have relied 

 much more upon a study of the sporangia and of the tissues as 

 disclosed by sections of petrifactions, and the results of these 

 studies have added very materially to our knowledge of the 

 affinities of the Ferns as gathered from a study of the structure 

 of the living species, and have throwai much light upon the his- 

 tory of the fossil forms. 



The earliest undoubted remains of Ferns occur in the Si-- 

 lurian. Of the few fossils of this age which can with reason- 

 able certainty be assigned to the Filicinese may be cited the 

 genus Rhodea, a Fern with finely dissected leaves, not closely 

 resembling any existing type. In the Devonian a number of 

 characteristic genera occur. Among these may be mentioned 

 Cardiopteris, Sphcnopteridium, Adiantites and Archcuopteris 



(Palcuopteris.) 



During the Carboniferous the Ferns increase rapidly in 

 number and variety, and constitute with the other Pterido- 

 phytes the predominant vegetation of the period. In the Sec- 

 ondary and Tertiary formations, they become less prominent, 

 giving way to the rapidly increasing Spermatophytes ; but 

 they have persisted to the present time in large numbers, and 

 have held their own much better than the other two classes. 



In studying the venation of the earliest Ferns, especially 

 the Arch^opteridse of Potonie, it is found that they all corre- 

 spond to a type found at present in comparatively few Ferns 



