236 CRETACEOUS PELECYPODA 



Shell solid, attached hy a portuju ul' oac valve, two large muscular impres- 

 sions, pallial Hue simple, a slielly ossicle attached to the valves by a cartilage close 

 to or behind the hinge. 



Chamostreji, Roissy, 1805. Lower valve attached on one side only, convex, 

 with the beak incurved, upper valve smaller, flattened ; shell inside slightly perla- 

 ceous ; hinge in the upper valve with a single tubercular tooth fitting into a pit 

 of the lower valve ; behind the tooth lies the ossicle, the cartilage being situated 

 in small pits in each valve. 



Comparing the animal and shell of Ch. ulbida with that of Chama their 

 relation is indeed striking, especially wlien we select such species as the fossil 

 Chama gryphina, Lam., which externally is entirely indistinguishable from Chavi- 

 ostrca. The situation and size of the muscular impressions are also very similar, 

 except that in the latter the posterior scar is far removed from the hinge, while 

 in Chama it lies always close to it ; the hinge-teeth of Chama are, of course, 

 constantly different ; they alw'ays want the cartilage and ossicle, bvit the narrow^ 

 and very thin almost linear ligament well agrees in some species of Chama with 

 that of Chamostrea ; however, as the animals of the former in the character of the 

 mantle margins and of the siphons also present considerable differences from those 

 of the latter, the separation of the tw^o families must be kept up. 



The Chamostreids form in many respects a transition from the Cbauidm to 

 the SiPPURiTiDJE, particularly to some ReqniemcB and others, which as to external 

 form and the flat upper valve appear very closely allied to Chamostrea. The 

 recent species has the outer surface of the shell lamellar and distinctly different 

 from the inner, but it is not fibrous as in Bequienia ; the presence of the ossicle 

 and the internal cartilage has, however, undoubtedly its true analogue in the 

 structure of the hinge of such species of Bequienia Lonsdalei, Sow^, sp. 



As for fossil species of Chamostrea we know as yet nothing, but it seems 

 doubtful whether the recent Chama; have been all properly examined as to their 

 internal structure. The only character which would entitle us to refer a fossil 

 species to Chamostrea is the presence of a corresponding groove or pit in both 

 valves, for it is not very probable that the ossicle will often remain preserved. Such 

 internal characters are, however, rarely traced out in fossil species. 



XXII. TumUij—HIBB VBITID^T^. 

 Nothing is known of the animals of this entirely extinct family as yet peculiar 

 to the cretaceous period. We can only presume from the relation of their shells to 

 some of those of Monopleura and Caprotina, and of these again to those of Chama, 

 that the animals were somewhat allied to the latter; and there can be little doubt 

 that such was really the case. But in several respects some of the IIippuritid^^ 

 exhibit such peculiarities that at the first sight they hardly appear to have a relation 

 to ordinary Pelecypoda. "When the internal structure of their shells has been more 

 carefully stiulied, we shall no doubt be better able to make somewhat more approxi- 

 mate comparisons between the present and the former family. As to thickness of the 



