MOLLUSCA GASTROPODA AND LAMELLIBRANCHIATA. 



By N. Annandale and Standby Kemp. 



INTRODUCTION. 



Our object in preparing this report has been, not to criticise genera and species 

 from a taxonomic point of view, but to discuss in relation to their biological environ- 

 ment the distribution of the forms that occur in the Chilka Lake, and thus to bring 

 the Mollusca, so far as possible, into line with the other groups dealt with in this 

 volume. So far as nomenclature is concerned, we have in most cases followed that 

 adopted by Mr. H. B. Preston in the series of papers contributed by him to the 

 Records of the Indian Museum between 1907 and 1916. Full references to these and 

 to other papers dealing with what von Martens calls the ' ' sub-marine ' ' molluscs of 

 the Indian coasts are given in the bibliography on p. 364. 



In the collections from the Chilka Lake that we have sent to Mr. Preston he 

 recognizes no less than 34 species of Gastropods and 45 of Lamellibranchs. Of 

 these he has described 42 species as new. We do not disparage his work, under- 

 taken as it has been with a purely conchological aim, in saying that we expect 

 that many of these species will ultimately prove to be no more than dwarfed or 

 distorted phases of molluscs that occur elsewhere in more normal conditions. Cooke 

 has pointed out with admirable clearness in his volume in the Cambridge Natural 

 History (vol. Ill, p. 82) that a naturalist's concept of species and varieties in 

 Mollusca must be profoundly modified by his point of view. Our point of view is 

 not Mr. Preston's, but unfortunately we lack his special knowledge. We have 

 therefore accepted his conclusions in so far as they do not run counter to the facts 

 we have observed in the field. 



In our Introduction to this volume we have dealt at length with the physical 

 conditions of life in the different parts of the lake-system and in particular with 

 the periodic changes in the salinity of the water. It will be as well, however, to 

 recapitulate briefly our statements on these points in so far as they influence the 

 distribution of the Mollusca. 



The whole of the lake-system is very shallow, rarely more than 2 fathoms in 



