286 



CRETACEOUS LAMELLIBRANCHIA. 



small part (Figs. 43, 48, 49) but in others a large part (Plate LI, fig. 2, Text-figs. 

 45, 46) of the entire shell. 1 Also the passage from one stage to the other is 

 sometimes gradual, so that the section of the shell forms a fairly uniform curve 

 (Fig. 48), but generally the change is abrupt, so that the later part of the valve is 



Fio. 42. — Inoceramus inconstans, sp. 7iov. The original of Inoceramus sp., Mantell, ' Foss. S. Downs,' p. 217, 

 pi. xxvii, fig. 9. Upper Chalk, Lewes. British Museum, No. •1705. Left valve and posterior view. 

 Natural size. 





\ 



m 



, 



Fig. 43. — Inoceramus inconstans, sp. nov. Zone of Holaster planus, Swaffham, Norfolk. Norwich Museum. 

 Left valve, and posterior and dorsal views of the same. Natural size. 



bent at an angle with the earlier part (Figs. 42, 43). The length of the hinge-line 

 in proportion to the height of the shell shows some variation and consequently the 



1 The larger specimen of J. latus, Sowerby ('Mia. Conch.,' vol. vi, pi. dlxxxii, upper figure), 

 which cannot now be found, is probably an example of this. 



