298 



CRETACEOUS LAMELLTBRANCHIA. 



shallow .sinus near the postero-ventral angle. Posterior margin nearly straight, 

 somewhat oblique, forming an angle with the ventral margin. A broad shallow 

 depression extends from behind the umbo to the postero-ventral sinuosity ; 

 posteriorly this depression is limited by an angular ridge. Behind the ridge is a 



Fig. 



-Jnocerainus lobaius, Orolilf. Zone of Actiiiocumax qiKulraliis, Yorkshire. York Museum. Part of 



left valve. x |. 



flattened or slightly concave wing-like part which, owing to the thinness of the 

 shell, is often not preserved. Umbones acute, near the anterior end. 



Ornamentation consists of concentric ril)S with an unsymmetrical curvature ; in 

 the posterior depression the ribs bend upwards, forming a sinuosity, and on the 

 wing the ribs bend upwards and are less distinct than elsewhere. The concentric 

 ril)s are of two sizes ; large ribs occur at intervals and between these are several 

 sinallcr ril)s. 



