INOCERAMUS. 



295 



Concentric ribs strong, sharp, narrow, with a very unsymmetrical curvature ; 

 a few ribs may come off from the sides of other ribs ; between the ribs are broad 

 concave interspaces. On the marginal parts of old specimens ribs are absent. 



Affinities. — I. balticus is closely allied to I. incovstans, from which it has 



> 





Fig. 53. — Inocennnus balticus, Bohm. Upper Chalk (probably zone of Marsupites testudinaritis), Brighton. 

 Museum of Practical Geology, No. 25509. Lett valve and dorsal view, x j. 



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

 longer shell, with the umbones not quite terminal. The length of duration of the 

 slightly convex stage varies in the same way as in /. inconstans. The specimen of 

 I. inconstans shown in Fig. 45 makes, on account of its longer hinge, some 

 approach to I. balticus. 1 



1 See also Bohm, loe. oil., pi. xii, fig. la. 



