igi6.] Fauna of the Chilka Lake : Mollusca Gastropoda , etc. 341 



Such forms as those of Kelly a, Scintilla, Diplodonta, Solen, Standella, Tlieora, 

 Citmingia, Cuspidaria, Lyonsia and Anatina are remarkable in this respect, while the 

 shell of the dementia is so fragile that we had great difficulty in preserving perfect 

 specimens. Thick-shelled species such as those of Area , Meretrix and Tapes are few 

 and have almost completely disappeared from the main area. Except possibly in the 

 case of Modiola, we have, however, no evidence that individuals from the lake have 

 thinner shells than those of the same species living elsewhere. Among the Gastro- 

 pods, Thais and Potamides are exceptional in the thickness of their shell ; there is no 

 form comparable in the opposite direction to dementia. 



The thinness of shell in the lake species can hardly be due to lack of dissolved 

 calcareous matter, for considerable quantities of ' kankar ' (nodular concretions of 

 carbonate of lime) are dug from the bed of the lake when the level of the water is 

 low. In the case of many of the lyamellibranchs (e.g. dementia and Theora) it is 

 associated with life in peculiarly soft and adhesive mud, through which the animals 

 progress with considerable rapidity. It is noteworthy, moreover, that all the thick- 

 shelled burrowing Mollusca found in the lake inhabit sand or sandy mud and that 

 there is no evidence that the shells of such forms are thinner than those found in pure 

 salt water. 



These facts are of some interest because instances are well known in the Baltic 

 and elsewhere, in which the shells of marine species related to the Chilka forms 

 become greatly attenuated in brackish water. Gibbons 1 has, however, pointed out 

 that though this is the general rule , the shells of true brackish water species may 

 tend to become thicker in correlation with decrease of salinity. 



We have already alluded to the fact that, especially in the main area, a com- 

 paratively small number of species predominate greatly in respect to number of 

 individuals. It is probable that if a census of the Mollusca of the main area could 

 be taken, the great majority would fall into some eight or nine species and some half 

 dozen genera. This feature is also characteristic of other groups of animals found 

 in the lake and, indeed, generally of animals living in abnormal conditions. 



LIST OF SPECIES. 



Class GASTROPODA. 



Order OPISTHOBRANCHIATA. 



Family Tornatinidae. 



Tornatina estriata, Preston, 1914, p. 303, figs. 7, ya ; 1915, p. 297; 1916, p. 27 

 (as Retusa); syn. T. soror, Preston, 1914, p. 3°3, fig s - 8, & a - 



This is one of the commonest Gastropods on a muddy bottom in both sections 

 of the lake-system. Shells from the outer channel tend to be a little larger than 

 those from the main area and to have a less ovately cylindrical form. Mr. Preston 

 separated the latter under the name T. soror in 1914, but has now found inter- 



1 Gibbons, Quart. Joum. Conch., I, p. 339 (1878.) 



