INOCERAMUS. 



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spiral; with a very large umbo near the anterior end, curved inwards an< 

 forwards. Surface nearly smooth, except for the presence of growth-lines. 



The hinge (Fig. 94) curves at either end; it is thinnest near the umbo am 

 becomes thicker towards the posterior end. The ligament-pits are deep, aim 

 square, but sometimes oblong, and increase in height from the umbo posteriorly 



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osi 



Fig. 92. — Eight valve and part of left valve of specimen shown in Figs. 90, 91. 



Affinities. — I. involutus is the type of the genus or sub-genus Volviceramus of 

 Stoliczka. In general appearance this differs considerably from other species of 

 Inoceramus, so that its separation as a distinct genus or sub-genus seems at first 

 sight quite justifiable; but the study of a large number of specimens of I. involutus 

 and I. LamarcJci shows that these two species are very closely allied, and that the 

 former has almost certainly descended from the latter. Such being the case it 

 follows that these two species are more nearly related to one another than are 



