OF SOUTHERN INDIA. 227 



3. Sippopiis, Meusch., 1787.* Shell quite similar to Tridacna, but the 

 surface is more tubercular, and the pedal opening is only indicated by a number 

 of small serrations of the margin. 



H. and A. Adams state that the hinge is composed of " two compressed, 

 unequal, primary teeth in each valve." There is, however, strictly speaking, no 

 difference in the hinge of Sippopus and Tridacna. In the former we have in the 

 right valve a rather long lamelliform cardinal tooth, the hinder part of which 

 continues as a raised inner edge of the hinge-margin and is sometimes broken up 

 into two or three lamellte ; besides this long cardinal tooth there are two posterior 

 but rather indistinct lateral teeth ; in the left valve the hinge-teeth are also very 

 similar to those of Tridacna, except that the furrow separating the two cardinal 

 teeth is continued posteriorly, and passes directly in the groove which bounds the 

 posterior lateral tooth below. 



With the exception of the peculiar palaeozoic Eurydesma we do not know any 

 fossil species of the Tridacxid^. Looking at the few recent forms, the character 

 of the animals with their united siphons and the solid shells with the peculiar 

 single cardinal tooth, fitting into a groove of the other valve, certainly indicate a 

 relation to the Csamid^. The position of the median large muscular impression 

 in tbe two recent genera is quite peculiar, but I think that the two smaller impres- 

 sions below the anterior and posterior terminations of the hinge-area are to be 

 more correctly identified wdth the two muscular scars of other Pelecyj)oda, though 

 that opinion has been contradicted by several conchologists. There can. be, how- 

 ever, no doubt that they serve for the attachment of similar muscles as in other 

 Pelecypoda. 



XX. Family,— CHA 3IID^E. 



The animals of the recent Chamidje have the inner mantle margins united, 

 the outer thickened, fringed and separated in their entire length ; there is a small 

 opening in front for the protrusion of a rather short, cylindrical, and usually 

 angularly bent foot, and another opening behind for the two siphons which are 

 separated, with fringed orifices, but very short ; the labial palps are small, nearly 

 quadrangular and obliquely truncated; one pair of gills on each side, unequal, 

 narrow, the outer half posteriorly prolonged and united. 



The shells are very solid, free, or with one valve partially attached to other 

 shells, rocks, or corals; the beaks spiral and more or less marginal, muscular 

 impressions two, large, pallial sinus entire ; hinge usually with one blunt cardinal 

 tooth in one valve, fitting between two more or less unequal teeth of the other 

 valve ; sometimes there is besides a small posterior lateral tooth present ; ligament 

 external, linear, generally situated in a groove running from the beaks to the hinge, 



* Meuschen must be quoted as the authority of the genus, for as all the other forms (but H. equimis), noticed 

 by Meuschen under the above name, have been placed by Lamarck in different genera, it must be understood that 

 he reserved the name for that single species. DaCosta already figui-es it as distinct from Tridacna, \>\\i ca\h it a 

 peculiar Cockle. 



3 K 



