210 STUDIES IN FOSSIL BOTANY 



In the first instance, the genus Sigillaria has been 

 divided into two main series, those with ribbed stems — 

 the Eu-Sigillariae, and those without ribs — the Sub- 

 Sigillariae. The ribs, characterising the former series, 

 are broad longitudinal ridges, each of which bears a 

 single vertical series of leaf- scars. The ridges are 

 separated by comparatively narrow furrows (Figs. 86 

 and 87). 



The Eu-Sigillariae were formerly divided into two 

 subgenera, Rhytidolepis and Favularia — names which, 

 though no longer of taxonomic significance, may be 

 retained to designate types of surface. In Rhytidolepis 

 the ribs, which are often much broader than the leaf-scars 

 borne upon them, are separated by straight vertical 

 furrows. The scars of the same vertical series are in 

 some cases almost contiguous ; more often they are 

 separated by considerable spaces (see Fig. 87). The 

 ribs have been looked upon as representing series of 

 fused leaf-bases, a view, however, which is negatived by 

 the observations of Arber and Thomas, who find that 

 the true leaf-bases form bracket-like projections from 

 the ribs, the latter being really part of the cortex. 

 Rhytidolepis is the most characteristic form of Sigillaria, 

 and the most remote in outward appearance from Lepi- 

 dodendron. The great ribbed stems of this type are 

 a striking and familiar feature of the Coal-flora, and 

 are among the best-known fossil remains. 



The other ribbed type of surface — termed Favularia 

 by Sternberg — has the ridges separated by zigzag 

 furrows, while a marked transverse furrow intervenes 

 between, the adjacent leaf-scars of the same series (see 

 Fig. 86). Here, in fact, each scar is seated on a well- 



