548 



CYCADOPHYTAN FEONDS 



[CH. 



the lower surface. There are approximately 120 stomata per 

 square miUimetre ; each with two subsidiary cells (fig. 609, C) 

 of elongated form, and the guard-cells have small thickened out- 

 growths or papiUae as in D. Johnstrwpi. 



The epidermal features are described under D. Johnstrwpi 

 and D. Hawelli (fig. 609, C) . a fuller account of those in the latter 

 species will be foxmd in the paper by Mr Thomas and Miss Bancroft. 



Fig. 609. Dictyozai/iites HnwelU. (A, B, British .Museum; C. after 

 Thomas and Bancroft.) 



PTEROPHYLLUM. Brongniart. 



The name Pterophyllum was first applied by Brongniart^ to 

 some fronds from the Lias of Hor, which he named P. majus 

 and P. minus, species subsequently removed by Nathorst^ to 

 Schimper's genus Anomozamites. Brongniart* defined Ptero- 

 fhyllum fronds, e.g. P. Jaegeri (fig. 610), as pinnate, bearing 

 pinnules of almost equal breadth attached by the whole width 

 of the base and with a truncate apex ; veins slender, equal, simple 

 and shghtly arched. There has been considerable confusion and 

 laxity in regard to the application of the name Pterophyllum 

 and in many cases no clear distinction has been recognised between 

 this genus and Nilssonia. In Nilssonia the distinctive feature 

 is the complete absence of any rachis uncovered by the lamina 



1 Brongniart (25). ' Nathorst (81) p. 61. '' Brongniart (28) A. p. 95. 



