190- MR. H. HAMSHAW THOMAS AND MISS N. BAKCEOFT ON 



J 



slightly thickened, and in many cases are not preseryed, hut the stomatal slit is 

 frequently visihle. 



r ^^^ 



Ctenis falcata presents a type of stomatal structure comparahle with that seen 

 in Nilssonia mediana. In the former species, however, there is little or no tendency 

 towards the papillate outgrowths from the subsidiary cells surrounding the stomatal 

 apertures, which is so pronounced a feature in some Nilssonias, for example N. compta. 



Ptilozamites (Ctenozamites) Leckenbyi (Leckenby, ex Bean MS.), n. comb. 



The cuticles of this form have already been figured by Prof. Nathorst (1908, Taf. 1. 

 figs. 3-5) as providing an example of a frond with stomata on one side of the pinn8& 

 only. He did not enter into a detailed description of the structures seen. 



In comparing the structure of the cuticle with that of other genera, the following 

 points may be noted. The leaves have a very stout texture with thick cuticles so that 

 the leaflets separate off whole from the matrix and can be easily handled. In most of 

 the specimens examined the serration of the margin is not pronounced, but a few teeth 

 are seen near the apex. The upper cuticle is very thick and shows very clearly the 

 outlines of the epidermal cells which must originally have had their radial walls very 

 considerably cuticularised. The cells are irregularly shaped, but are straight-walled 

 and somewhat rectangular. The cells above the nerves are more elongated than 

 the rest. The lower cuticle is also thick and shows cells similar in form to those of the 

 upper epidermis. The cells are sHghtly elongated above the veins, but the courses of 

 the hitter are not clearly defined and are not always devoid of stomata. The stomata 

 were numerous, their distribution being about 90 per square mm. (PI. 20. fig. 12). In 

 form they show considerable resemblance to those in Ctenis, being slightly sunk, and 

 having the w-alls and opening of the stomatal cavity much thickened (PL 19. fig. 17) ; 



the cuticle round the opening, in fact, forms a little ring, slightly raised beyond the 



urface of the pinna. The seven or eight subsidiary cells are somewhat thickened and 

 appear as a group darker than the rest. On mounting a lower cuticle so that the 

 mesophyll side is uppermost, the outlines of the guard-cells are faintly seen ; these 

 appear to have some resemblance to those of the Williamsonia-Taeniopteris group. The 

 slit lies at right angles to the longer diameter of the stomatal cavity, and on either side 

 of it there are distinct though faint indications of guard-cells (?) with pointed ends 

 (PI. 19. fig. 16). 



(o) The Cuticles of some other Types of Fossil Oycadean Fronds. 



The cuticular structure of some other genera have been described bv other authors, 

 but in several cases the figures given are not sufficiently detailed or accurate to allow of 

 their close comparison with the examples here described. Prof. Nathorst has figured 

 and described the cuticles of tw^o species of I'seudocycas from the Lower Cretaceous 

 (1907, pp. 7, 9, Taf. 2. figs. 1-11). These cuticles show more or less rectangular cells 

 arranged in regular rows and having undulating walls. The stomata are not clearly 

 recognisable, but from Taf. 2. fig. 11, they appear to be somewhat similar in general 



