igi6.] Fauna of the Chilka Lake : Mollusca Gastropoda, etc. 335 



the coast of Orissa by the S.S. ' Golden Crown,' but not a single species is common- 

 to this collection and to our own, while most of the genera are different, — a fact due 

 perhaps in the main to the nature of the bottom on which the two collections were 

 obtained. 1 So far as we have been able to discover, the molluscan fauna of the Chilka 

 Lake, at any rate in the matter of Lamellibranch genera, is nearer to that collected, in 

 shallow water and mainly on muddy ground, by the Danish Expedition to Siam 2 than 

 to any other on which a comprehensive report has yet appeared. Eighteen Lamelli- 

 branch subgenera and genera are common to the two collections, representing two 

 thirds of those found in the lake. 



BIOLOGICAL DISTRIBUTION. 



Less than 50%. of the living Mollusca of the lake are found in the main area, 

 and even this percentage is somewhat reduced if we omit the island of Nalbano. 

 With two exceptions, viz. Corbula chilkaënsis and Chrysallida ecclesia, each repre- 

 sented by a single specimen, all species found in the main area were also found in 

 the outer channel, but the great majority did not occur on the clean sandy bottom of 

 the seaward part of the latter. By far the richest tract in the whole lake-system is 

 the southern end of the outer channel between Barhampur I. and Satpara Point 

 (see map, PI. II of this volume). 



The following species have a great numerical preponderance throughout the 

 main area , except where the water is excessively shallow : — 



Gastropoda. Lamellibranchiata. 



Tomatina estriata. Modiola undulata. 



Nassa denegabilis. dementia annandalei. 



,, orissaënsis. Solen ? fonesi. 



Stenothyra spp. Theora opalina. 



With the exception of the species of Nassa and Stenothyra, all of these are much 

 less abundant in the outer channel. In the channel the following species may per- 

 haps be regarded as predominant : — 



Gastropoda. Lameujbranchiata. 



Nassa labecula. Meretrix casta. 



Potamides fluviatilis. ,, ovum 



Litiopa copiosa. Tapes pinguis. 



Pyrgulina humilis. ,, ceylonensis. 



In this part of the lake it is much more difficult to select predominant species 

 than in the other, for a large number of forms are represented by considerable 

 numbers of individuals, whereas in the main area most of the species are either very 

 rare or else extremely abundant. 



1 Jenkins, Rec. Ind. Mus., VII, p. 51 (1912). 



* Lynge, Danske Vid. Selsk. Skrijt. (7) Nat. og. Math., V, pp. 100-299 (1909). 



