344 FILICALES [CH. 



Hutton^ is Brongniart's Cladophlebis denticulata. It is im- 

 possible to determine with accuracy the numerous examples 

 described as Pecopteris whitbiensis, Asplenium whitbiense, 

 Cladophlebis Albertsii (a Wealden species''), Asplenium, or 

 Cladophlebis, nebbense^, etc., from Jurassic and Rhaetic strata. 

 The Cladophlebis denticulata form of frond is one of the 

 commonest in recent ferns ; it is represented by such species as 

 Onoclea Struthopteris, Pteris arguta, Sadleria sp., Oleichenia 

 dubia, Alsophila lunulata, Cyathea dealbata, and species of 

 Polypodium. It is, therefore, not surprising to find records of 

 this Mesozoic species from many localities and horizons. All 

 that we can do is to point out what appear to be the most 

 probable cases of identity among the numerous examples of 

 fronds of this type from Mesozoic rocks, particularly Rhaetic 

 and Jurassic, in different parts of the world. The name Clado- 

 phlebis denticulata may be employed in a comprehensive sense 

 for fronds showing the following characters : — 



Leaf large, bipinnate, with long spreading pinnae borne on a com- 

 paratively slender rachis. Pinnules, in nearly all cases, sterile, reaching a 

 length of 3 — 4 cm., acutely pointed, finely denticulate or entire, attached 

 by the whole of the base (fig. 257). In the apical region the pinnules 

 become shorter and broader. Venation of the Cladophlebis type (fig. 256, A). 

 Fertile pinnules rather straighter than the sterile, characterised by 

 linear sori parallel to the lateral veins (fig. 258). 



In endeavouring to distinguish specifically between fronds 

 showing a general agreement in habit with C. denticulata, 

 special attention should be paid to venation characters, the 

 shape of the pinnules, the relation of the two edges of the 

 lamina to one another, and to the amount of curvature of the 

 whole pinnule. Unless the material is abundant, it is often 

 impossible to distinguish between characters of specific value 

 and others which are the expression of differences in age or of 

 position on a large frond, to say nothing of the well-knewn 

 variability which is amply illustrated by recent ferns. It is 

 remarkable that very few specimens are known which throw 

 any light on the nature of the fertile pinnae. Fig. 258 repre- 



1 Lindley and Hutton (34) A. PI. cxxxiv. ^ Seward (942) jj^_ p_ gi 



' Nathorst (78). 



