228 CEETACEOUS PELECYPODA 



a portion of the ligament is often internal ; the presence of a cartilage seems in 

 some forms to be also indicated. 



The solidity of the shells, spiral beaks, large mnscular impressions, and the 

 presence of one or two very strong cardinal hinge-teeth are the prominent charac- 

 teristics of this family. Considering these, I think the family must receiye a 

 larger extension, than is usually giyen to it in conchological works. BiGerocardkim 

 Lycodus and Diceras should certainly be classed together with the ChamcB. The 

 first is a triassic form, the second probably liasic, the third Jurassic and cretaceous. 



In this last formation several other new forms appear, which in all the principal 

 characters agree with Chamce, as was some years ago pointed out by S. P. Wood- 

 ward (Man. of Moll., 2nd ed., p. 37), whose classification I will in the main 

 follow. The genera to which I allude are Monopleura, Bequienia, Caprina, Capri' 

 nula, &c., &c., some of which are said to possess also an internal ligament, and if 

 this be really the case those forms will have to be placed in the family Ckamostrjeibm, 

 the relation of which to the present family will be pointed out further on. The 

 HippuRiTiDjE (as restricted) will also form a distinct family from the present one, 

 in which I will include all the forms with one valve spiral and the other more or 

 less flattened and with concentric striae of growth. 



The shells of the recent and fossil Ohamce consist, as noted by Woodward, 

 H. and A. ildams, and others, of three distinct layers; 1^^, the outer, which is 

 secreted by the external, free, and thickened portions of the mantle; it has a 

 tubular structure and is deposited in separate layers, both these circumstances pro- 

 ducing a peculiar reticulated appearance, so well known and formerly believed to 

 be entirely peculiar to the shells of Hippurites. In the ChamcB the tubular struc- 

 ture is well seen in semi-decomposed shells, but the reticulated structure is very 

 fine and in recent shells only traceable on a microscopical section. All the external 

 ornamentation, with the spines, lamellae, &c., and in fact the greater part of the 

 thickness of the shell, is exactly as in Hippurites, I shall speak of this outer layer 

 as the reticulated one. 



"Ind. — The middle layer is semi-transparent, opaque, of a silky appearance, and 

 consists of vertical, prismatic cells. It varies in thickness in different species, 

 and is sometimes very thin ; it is only secreted within the extent of the pallia! 

 impression. I will call this the prismatiG layer. 



Zrd. — The innermost is the thinnest layer, of a pearly or glassy appearance, 

 milky or semi-transparent, and rather homogenous ; this is the glassy layer ; it 

 appears to become obsolete or is not preserved in most fossil species. 



In some tertiary species, as, for instance, Gh, gryphina, Lam., these layers can 

 very well be traced. In a comparison of the recent ChamcB with the fossil 

 CaprotincB and ILippiirites the distinctions of these three layers will be of very 

 great importance, as I shall notice further on. 



The recent Chamce chiefly include tropical forms ; they are, strictly speaking, 

 littoral shells, living attached to corals and rocks, and many of them are out of 

 water during low tides. Eeeve describes 55 species in his Monograph, and but 



