XXXIX] CLASSIFICATION 511 



the Cycadophyta, in other cases it is possible to assign fronds 

 to a section of this comprehensive group characterised by a 

 particular type of fertile shoot and by certain well denned epi- 

 dermal features. 



The investigation of the cuticular structure of various Cycadean 

 fronds by Nathorst 1 and especially by Mr Thomas 2 has supplied 

 a basis of classification which affords the best criterion of affinity 

 so far available. The majority of fronds are placed in the Bennet- 

 titales while the three genera Ctenis, Nilssonia, and Ctenopteris 

 (or Ptilozamites 3 ) are placed in the Nilssoniales. 



I. Bennettitales. Epidermal cells characterised by sinuous 

 walls and generally rectangular ; the cuticle is thin ; the stomata, 

 confined to the lower surface of the pinnae, tend to be arranged 

 at right-angles to the veins and are on a level with the epidermis 

 or very slightly depressed; two large laterally placed subsidiary 

 cells more or less surround the guard-cells and these are provided 

 with thickenings of a definite shape (figs. 594, 609). 



Genera : Ptilophyllum, Zamites, Otozamites, Dictyozamites , 

 Pterophyllum and Anomozamites, Taeniopleris, Pseudocycas. 



II. Nilssoniales. Epidermal cells with straight walls, not 

 sinuous, irregular in form, rounded, hexagonal, or rectangular; 

 the stomata are below the level of the epidermis; the cuticle 

 may be thin or thick. There is no regular arrangement of the 

 stomata; the guard-cells are surrounded by 6 8 subsidiary 

 cells (fig. 625) which often form an overarching canopy ; thickening 

 lamellae like those on the guard- cells of the Bennettitales are 

 seldom present. 



Genera: Nilssonia, Ctenis, Ctenopteris. 



It is a noteworthy fact that the representatives of the smaller 

 group, the Nilssoniales, in their cuticular features, are more akin 



1 Nathorst (12 3 ) p. 36. 2 Thomas and Bancroft (13). 



3 The genus Ptilozamites described briefly in Volume n. has recently been fully 

 investigated by Antevs* who had access to the rich material in the Stockholm 

 Museum from the Rhaetic beds of Scania. He thinks that the genus is intermediate 

 between Anomozamites and Ctenopteris-, its systematic position has not been 

 definitely established but, as Antevs says, there seems to be no reason why it 

 should not be a member of the Cycadophyta. In some species, e.g. Ptilozamites 

 fallax Nath. and P. Nilssoni Nath., the rachis is forked, while in P. Heeri Nath. 

 with linear fronds reaching a length of 53 cm. the rachis is undivided. 



* Antevs (14 2 ). 



