INOCERAMUS. 



295 



Concentric ribs strong, sliarp, narrow, witli a very unsymmetrical curvature ; 

 a few ribs may come off from the sides of otlier riljs; between tlio ribs are broad 

 concave interspaces. On the marginal ])arts of okl specimens ribs are absent. 



Affinities. — /. hiillirns is closely allied to f. inronsfuns, IVom whic-li it lias 



' ) 



X 



\ 



\ 



Via. 53.— /iiofei-aniiis hiilticus, Buhiii. Upjx^r Cliiilk (proliiiWy zone of Mannpiles testudinarius), Brighton. 

 Mtisoiiiii of I'nioticiil Oeology, No. 2.).jO!». I,i>ft valve .iiul dorsjil view, x }. 



probably been derived; it differs from that species by its longer hinge and the 

 longer shell, with the umbones not quite termin;il. The length of duration of the 

 slightly convex stage varies in the same way as in /. incini.'itdii.t. The specimen of 

 /. iiu-oiisftin.'i .shown in Fig. 40 makes, on account of its longer hinge, some 

 approach to /. balticus.^ 



' See also Bubiii, loo. cit., pi. xii, fig. Id. 



