298 



CRETACEOUS LAMELLIBRANCHIA. 



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. 55. — Inoceramus lobatus, Goldf. Zone of Actinocamax quadratus, 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 ribs with anunsymmetrical 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 

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

 smaller ribs. 



