SIGILLARIA 



227 



tions of the primary zone. This latter character may- 

 serve to distinguish a Sigillaria from a Lepidodendron, 

 but too much stress must not be laid on it. The inner 

 cortex is imperfectly preserved ; the outer bark consists 

 chiefly of a thick zone of radially seriated periderm 

 (Fig. 93,f>d), beyond which a narrow band of primary 

 cortex (c) abuts on the surface of the ribs. The leaf- 

 traces are ranged in vertical rows corresponding to the 



Fig. ^.Sigillaria (Jikytidohpis type). Segment of stem, in transverse section, includ- 

 ing three ribs, x, crenulated primary wood ; jr 2 , secondaiy wood ; £d, periderm ; c, 

 outer cortex. X 3. From a photograph by Mr. A. Gepp. 



ribs of the cortex. The xylem-strands of the leaf-traces 

 start from the depressions of the crenulated primary 

 wood, just as in Sigillaria Menardi or spinulosa, and in 

 Williamson's specimen mentioned above. It is thus 

 evident that each concave segment of the continuous 

 primary wood, in the Rhytidolepis type of stem, corre- 

 sponds to one of the distinct bundles, as shown in the 

 Sub-Sigillarian forms previously described. 



A more perfect specimen of the stem of a ribbed 



