190 MR. H. HAMSHAW THOMAS AND MISS N. BANCROFT ON 



II. Nilssoniales.* 



NiLSSONiA, Brongn. 



While the cuticles of the genera already described show a very considerable agree- 

 ment, both in the structure of their epidermal cells, and also in the form and arrange- 

 ment of the stomata, the cuticles of the remaining genera Nilssonia, Ctenis, and 



Ftilozamites 3.YQ qmie distinct from them. In these forms the epidermal cells are some- 

 what irregular in shape, being rounded, hexagonal, or rectangular; the walls are straight 

 without any indication of folding. The stomata appear to be somewhat sunken, and 

 when auy trace of the guard-cells can be seen, they apparently lack special thickening 

 and have a more or less rounded contour. The appearance of these cuticles therefore 

 marks them at once as distinct from those of the Bennettitalean group. 



The Nilssonias must have been important constituents of the Jurassic vegetation of 

 the Yorkshire area. Fronds of Mhso7iia compta occur abundantly in the Middle 

 Estuarine bed in the neighbourhood of Gristhorpe Bay. In the same locality 

 N. mediana occurs : this is almost identical in form with a plant which is very abundant 



i- 



in the Lower Estuarine beds of Marske and Boseberry Topping, but which possesses 

 very much more delicate cuticles. The same form occurs at Whitby, while another type, 

 closely allied to N. compta, but probably a species new to England, is found at Boseberry 

 Topping. The cuticles of these three species or forms have been examined, and their 

 characters are briefly described below. The cuticles of the Swedish species hrevis, 

 polymorpha, and pterophylloides have been described and figured by Prof. Nathorst. 

 They apparently agree somewhat closely with the English species described here, though 

 little or nothing could be made out about the form of their stomata. 



NlLSSONIA COMPTA, Phill. 



The examples of this species from the Gristhorpe bed indicate that they originally 

 had a somewhat delicate texture, and owing to this and to the thinness of the cuticles it 

 is not easy to obtain good cuticular preparations from them. Even after portions of 

 the carbonised lamina have been successfully removed from the matrix they frequently 

 break up into numerous small fragments during the clearing process. It is believed 

 that tids affords a character of real value in describing and identifying fronds. In many 

 descriptions of leaf -impressions, the lamina is described as leathery or coriaceous, from 

 the appearance of the specimen when still attached to the rock or even from the 

 appearance of its cast ; in most of these cases little value can be assigned to such a 

 statement. When, however, we are able to remove the carbonised or mummified plant 

 from the matrix and investisjate the structure of its cuticle, we have definite grounds 

 for a statement as to the comparative texture of the lamina. 



In the case of the plants preserved in the fine-grained mudstone of the Gristhorpe bed, 

 wo can say with some confidence that such fronds as Toeniopteris vittata, T. majors 

 Ctenis falcata^ and especially JPtilozamites LecJcenhyi, had a firm and coriaceous lamina. 

 Others, such as Anomozamltes Nilssom and Nilssonia compta^ were of intermediate 



* See page 196. 



