DIOONITE3. 41 



has been found useful by several writers as a convenient name 

 to apply to Indian fronds, but as at present used it does not 

 appear to be wholly satisfactory. The genus Ptilophyllum seems 

 to have been almost confined to Asiatic fronds, and the locality 

 of a specimen has probably had too great a share in the 

 selection of Ptilophyllum in preference to Ctenophyllum as the 

 most suitable name. Nathorst * figures and describes a leaf 

 fragment from Japan as Ptilophyllum cf. cutchense, Morr., but 

 it would seem practically impossible to separate such a form as 

 this from some English Jurassic fronds usually placed in the 

 genus Ctenophyllum. 



On the whole perhaps the better course is to retain, at least 

 for the present, the name Dioonites as the most suitable generic 

 designation for the Wealden species D. Dunkerianus (Gb'pp.). 

 We must slightly modify the definition of the genus, and no 

 longer insist on the decurrent pinna base as an essential charac- 

 teristic. The implied relationship to the recent Dioon is the 

 least satisfactory feature of Dioonites, but possibly we shall be 

 able, on a future occasion, to suggest some further alteration in 

 the existing nomenclature of fossil cycadean fronds. We may 

 define this genus, using the term Dioonites in a wide and 

 provisional sense, as follows : 



Frond pinnate, pinnae at right angles, or more or less obliquely 

 inclined to the rachis, attached to the upper surface of the frond 

 axis, bases separate, may or may not be decurrent, not narrowed 

 towards the point of attachment, apices acuminate, straight or 

 slightly truncate, veins parallel. 



In dealing with Dioonites, as with many other genera, we 

 may easily fall into the error of excluding or including certain 

 forms owing to our imperfect knowledge as to the manner of 

 attachment of the pinnae ; but it is obviously impossible to 

 devise a perfectly satisfactory system, so long as we are limited 

 by the exigencies of fossilization and the imperfection of the 

 frond fragments. 



1 Nathorst (A. 3), p. 52, pi. iv. fig. 8. 



