528 CYCADOPHYTAN FRONDS [CH. 



afterwards figured by Dunker 1 . Miquel 2 included the species 

 in his genus Dioonites and this name has been adopted by other 

 authors. Attention has been drawn to the inconsistent use of 

 the title Dioonites 3 , and I have previously employed the name in 

 a sense similar to that in which it has been adopted by Nathorst, 

 that is for fronds with long and narrow pinnae without any basal 

 constriction and not auriculate, attached more or less at right- 

 angles to the upper face of the rachis; but so defined Dioonites 

 differs in no essential particular from forms of Ptilophyllum or 

 from fronds referred by authors to Schimper's genus Ctenophyllum. 

 The name Ctenophyllum* was instituted for certain fronds differing 

 in some points from Otozamites and Dioonites. One such type is 

 Ctenophyllum (Ptilophyllum) pecten (Lind. and Hutt.) : this is 

 quoted in Zittel's Handbuch as a typical representative of the genus. 

 Schimper followed Feistmantel in his definition of Ptilophyllum, 

 a definition which is not in accordance with the characters of the 

 fronds on which it was founded by Morris. Fontaine, on the 

 other hand, has applied Ctenophyllum to fronds of a different 

 type which are now included in the genus Pseudoctenis. There 

 would seem to be no adequate ground for the retention of Cteno- 

 phyllum as a generic designation. 



The Wealden species P. Dunkerianum is characterised by the 

 following features : rachis fairly stout, pinnae approximate, 

 linear 2 3 mm. broad and reaching a length of 11 cm. or more, 

 gradually narrowed towards the apex, attached in two almost 

 contiguous rows to the upper face of the frond-axis: the lower 

 margin of the lamina may be slightly decurrent, e.g. in the apical 

 part of the frond or very slightly broadened and bluntly rounded. 

 The pinnae are attached at right-angles or, near the apex, obliquely ; 

 veins 5 6, parallel. The epidermal cells as figured by Schenk 5 

 have very sinuous walls and are identical with those of the English 

 and Indian forms of the group-species P. pecten ; the stomata are 

 confined to the lower surface. If Schenk's drawings of the stomata 

 are correct they differ from those of other species of the genus 

 in their simpler structure; there are no subsidiary cells and the 



1 Dunker (46) A. p. 14, PI. n. fig. 3; PL vi. fig. 4. 



2 Miquel (51) p. 212. 3 Seward (95) A. p. 38. 



4 Schimper (70) A. p. 127. 5 Schenk (71) B. PI. xxxvi. 



