SIGILLARIA 211 



marked hexagonal leaf-cushion, though the arrange- 

 ment in vertical series is still manifest. The extreme 

 forms of Rhytidolepis and Favularia are distinct enough, 

 but both may occur on different parts of one and the 

 same stem, connected by intermediate forms, so no 

 systematic value can be attached to the distinction. 

 Generally speaking, the Eu-Sigillariae are geologically 

 more ancient than the Sub-Sigillariae, and some of the 

 oldest stems known show the Favularia type of surface. 



The Sub-Sigillariae, which are characteristic of 

 the Upper Coal-measures and Permian, agree in the 

 absence of distinct ribs, and have likewise been grouped 

 under two subgenera, to which the same remarks apply 

 as in the case of the previous group. In the Clathraria 

 of Brongniart, the surface of the stem is formed by 

 somewhat prominent, contiguous leaf-cushions, separated 

 by oblique furrows, each cushion bearing the scar of a 

 leaf (see Fig. 89, which, however, is too much like a 

 Favularia). Here the oblique parastichies stand out 

 more conspicuously than the vertical orthostichies, and 

 there is a certain approach to the Lepidophloios form of 

 surface, from which, however, the typical Clathraria 

 differs in the greater size of the scar relative to the 

 cushion, and in the form of both. 



In the other division of Sub-Sigillariae, the Leioder- 

 maria of Goldenberg, there are no prominent leaf- 

 cushions ; the scars are remote from each other, and 

 separated by a smooth cortical surface (see Fig. 88). 

 This form shows some approximation to Bothrodendron, 

 which has even been united with this group of 

 Sigillariae by some recent authors. The Clathrarian 

 and Leiodermarian sculpturings may both occur on the 



