OP SOUTHERN INDIA. 223 



V. Order,— CHAMACEA. 



The families which are here included in a seiiarate order under the above name 

 are five, the Verticordiid^, Tridacnid^, CaAniiDM, Grauostreidm, and Kippuri. 

 TiDM. With the exception of the last, each has, at least, one living representative, 

 though the whole order appears to have had its greatest development durin"- the 

 later times of the mesozoic period. Several important characters in the organisation 

 of the animals and of the shells make this oi'der a very natural group of Pelecypoda. 

 The animals are distinguished by their thick, generally rounded form, with a 

 double edge to the mantle, the inner one of which is united, the outer free, thickened, 

 and often provided with, or produced into, different kinds of appendages or filaments. 

 The inner united mantle has one opening for a small, more or less rudimentary 

 foot, and two for the siphons, which are slightly, or not at all, produced, but always 

 separated and often with one or both of theu' orifices fringed. The thickened free 

 edges of the mantle secrete the usual manifold sharp ornamentation of the shells 

 which possess a reticulated structm'e, and are often traversed by various anastomos- 

 ing canals. The gills and labial palps are usually of small size ; in the former the 

 inner pair of gQls is generally smaller than the outer, and sometimes nearly 

 obsolete. 



The shells are solid, thick, generally inequivalve, the larger valve being usually 

 partially attached, and one or both are richly ornamented with lamellse or spines. 

 The hinge consists generally of few large teeth, the muscular impressions are often 

 on elevated ridges and of great size, the pallial impression entire (or ? sinuated in 

 Hippveitidm) ; the ligament external, sometimes obsolete ; an internal cartilage is 

 present in some families. 



The Ceamidm which at present form the most extensive family of this order have 

 often been, more or less, closely associated with the Cardiidje, but there is actually 

 very little resemblance between them, except that the rich spiny ornamentation 

 is often common to both. The organisation of the animals with their united 

 mantle margins separates them widely from all the VENEBA CEA, and equally so 

 the structure of the outer layer of the shells. It is indeed very difficult to assign 

 to the present order a natural jjlace in a systematic arrangement of tlie Pelecy- 

 poda. The animals appear to combine the characters of the 3IYACEA and 

 VENEBA CEA, possessing the united mantle and small foot of the former, and the 

 short separated siphons of the latter. But as the former character seems to me 

 to be the more important one upon which our system should be based, I think 

 the relation to the first family is greater than to the second. 



The shells of the other families are by the small number and the arrangement 

 of the hinge-teeth, by the frequent occurrence of an internal cartilage, and the 

 occasional internal pearly structure also in favor of this classification ; and thus, 

 properly speaking, the CILAMA CEA could be considered as abnormal forms of the 

 MY A CEA, especially recalling some of the large Corhulm and allied genera. Were 

 it not apparently very unnatural to break up the series of connectiug links 



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