40 DIOONITES. 



the generic characters of the plant, to which he applies the name 

 Zamiophyllum Buchianum (Ett.), refers to Miquel's genus Dioonites 

 as characterized by the attachment of the pinnaB to the upper 

 surface of the rachis, and by the insertion of the segments almost 

 at right angles to the axis; he says nothing as to the decurrent 

 bases of the pinnaB. If we accept this definition, and depart 

 from the usually accepted feature of a decurrent pinna base, we 

 may well include the Wealden plants under this genus. It is 

 certainly not an easy matter to draw a definite line between 

 pinnae attached to the rachis by the entire base, which is not 

 decurrent, and those which are similarly attached, but with their 

 bases more or less decurrent. In the English examples of the 

 species D. Dunkerianus (Go'pp), the pinnse towards the upper 

 end of the frond are distinctly decurrent, but those occupying 

 a lower position cannot be described as possessing decurrent bases. 

 Cf. PL II. Fig. 3, and PL III. Fig. 6. There are two other 

 genera to which reference should be made, which to a certain 

 extent agree in their definitions with such fronds as D. Dunkeri- 

 anus, etc., viz. Ctenophyllum and Ptilophyllum. The former 

 genus was instituted by Schimper J to include certain forms of 

 fronds which do not in all essentials comply with the definitions 

 of Otozamites on the one hand, and Dioonites on the other, 

 PteropJiyllum pecten, L. and H., being taken as the type species. 

 The author of the genus afterwards somewhat modified his original 

 diagnosis, and pointed out that Pterophyllum Braunianum, Go'pp., 

 had been erroneously described as a species of Ctenophyllum. The 

 genus Ptilophyllum, proposed by Morris in 1840 2 for certain Indian 

 fronds, can with difficulty be distinguished from Ctenophyllum. 

 It is thus defined: 



"Fronds pinnate; pinnaa linear, closely approximated, more or 

 less elongate ; base variable in form, oblique, round, imbricate, 

 sometimes auricled in the upper and sometimes in the lower part. 

 Veins slender, equal, parallel." 3 



Gb'ppert long ago expressed the opinion that Morris' term 

 was a needless addition to the list of cycadean genera. 4 It 



1 Lot. fit. vol. ii. p. 143. 



2 Morris (2), p. 21. 



3 Morris (1), p. 116. 

 * (l),p. 117. 



