154 Records of the Indian Museum. [Voi,, XIII, 



made specially for this enquiry at Sonapur (Ganjam district), 

 Pulicat Lake, Palk Bay, Tuticorin, and the Tambraparni delta, 

 and from the prolific backwaters of the Malabar district where these 

 shells flourish in greatest abundance. 



The chief conclusions at which I have arrived are that no mul- 

 tiplicity of species exists ; that there are indeed only three really 

 good species of Mereirix living in the waters of Continental India, 

 namely M. meretrix, M. attenuata, and M. casta, and that while 

 the two former exhibit great variation in colouration, they are re- 

 markably stable in size and shape when mature, whereas the third 

 species exhibits a marked susceptibility to the mfluence of envi- 

 ronmental conditions, resulting in the production of numerous 

 varieties and local races. The conditions in east coast backwaters 

 being very different from those on the west coast, it results that 

 the main varieties of M. casta are similarly divergent, those of the 

 east coast being usually true to the type within narrow limits, 

 whereas those of the west coast, even when living within the same 

 estuary, may exhibit as many as three well-marked variations, 

 connected by a host of intermediate forms, merging so insensibly 

 into one another that it is practically impossible definitely to allo- 

 cate many to one particular group. 



Another notable fact brought out is the peculiar discontinuity 

 of distribution shown by one of the species [M. attenuata) and by 

 one variety of another (M. casta var. ovum). The former is known 

 only from specimens from the Nicobar Islands and from Gwadar, 

 on the Baluchistan coast ; whereas in the case of M . casta ovum, 

 we find it (a) widely distributed in the backwaters of the west 

 coast of India, and (b) on the west coast of the Malay Peninsula 

 and in Arakan. What may be the explanation in the case of M. 

 attenuata we cannot at present say : possibly it is a decadent form 

 once more widely distributed ; if so, palaeontology may be able to 

 assist. With regard to M . casta ovum the similarity of climatic 

 and physical conditions between the west coasts of India and the 

 Malay Peninsula probably supplies the reason. Between these 

 two localities lies the east coast of India, an area differing greatly 

 in climate, particularly in rainfall, from either of the other two. 

 Hence I believe that the formation of the varietal form is due to 

 the influence of divergent environment. That in two widely sepa- 

 rated localities, having however similar ph^^sical features and 

 climate, a parallelism of form should be maintained by the variety 

 is significant, and seems to be an instance where the influence of 

 similar environment in the production of identical varieties is in- 

 dicated. 



The following key to the Indian species and varieties sum- 

 marises my conclusions : — 



GENUS MERETRIX. 

 I. Pallial sinus very shallow and without an acutely projecting ventral horn. 

 A. Anterior cardinal tooth of left valve distinctly notched ; size of shell 

 large, usually attaining over 60 mm. in antero-posterior length. 



