CUTICLES OF CYCADEAN EEONDS. 



191 



character, while Wilssonia mediana and N. orientalis had a distinctly delicate or 

 thin lamina, approaching the type possessed hy the ferns and seldom separating from 



the matrix in an entire condition. 



The upper epidermis of N. compta is very distinct from the lower. It possesses small 

 rather irregularly-shaped cells arranged, to some extent, in rows. The individual cells 

 vary greatly in shape ; they may he i so diametric with rounded or angular outlines, or 

 they may be more elongated, and of rectangular to fusiform shapes. The latter kind 

 probably occur above the nerves. All the cell-walls are straight and not sinuous. 

 There are usually no stomata present, but in one or two cases, structures which may be 

 stomata, are seen near the margin of the lamina. A few rounded papilla) occur, these 

 being of the type common to most of the Nilssonias. Their form is better seen on the 



rachis and in other species. 



The cells of the lower epidermis are much more clearly seen ; the nerve-courses are 

 distinctly marked by the occurrence of rows of elongated cells, often bearing papilla?, but 

 devoid of stomata. The spaces between the nerves are occupied by rounded cells, with a 

 good deal of thickening in the corners. The cells surrounding the stomatal apertures 

 are much thickened. The stomata on these leaves are not numerous ; estimates from 

 the very fragmentary cuticles indicate between 40 and 50 in each sq. mm. It is almost 

 impossible to make out the exact structure of the stomatal openings, but the following 

 features are recognisable. Each aperture is surrounded by crushed thickened cells, which 

 evidently formed, in their original condition, a chimney-like structure, being raised 

 beyond the surface of the leaf, and enclosing a pit, at the base of which the guard-cells 

 were placed. Amongst recent Cycadean cuticles, a similar condition is seen in Bioon 



and in Cy 



The fossil cuticles do not show the projection of the cell 



rounding the stomatal apertures, owing to crushing in preservation. The cells are 

 seven or ei«'ht in number, and often show a star-like arrangement. Their relations to 

 the ordinary epidermal cells are not clear. When some of the preparations are mounted 

 so that their inner side (viz., that abutting on the mesophyll) is uppermost, the remains 

 of the guard-cells can often be distinguished (PI. 19. figs. 14 & 18). The guard-cells 

 themselves may be somewhat indefinite, but the pore is usually quite distinct. The 

 thickening of the guard-cells seems to be of a simple type, and not complex as in the 

 Bennettitalean fronds ; this may perhaps be correlated with their sunken position. 



The cuticle from the rachis below the frond presents some distinctive features. It is 

 composed of elongated spindle-shaped cells, giving the appearance which sclerenchy- 

 matous cells show in longitudinal section. At frequent intervals occur oval cells, which 

 are considerably more cuticularised than the rest. In the centre of each of these is a 

 rounded area, probably representing a short papilla. Structures of this nature are also 

 found sparingly on both surfaces of the lamina, as mentioned above. 



NiLSSONiA MEDIANA (Lcckcnby, ex Bean MS.), n. comb. 

 Examples of this species from Gristhorpe do not yield good preparations owing to the 

 delicate nature of the cuticle ; specimens from Marske, which are very similar in external 

 form, have a well-thickened cuticle, and preparations from these show the outlines of 



