334 Memoirs of the Indian Museum. [Vol,. V, 



Several of the genera represented in the lake fauna, though essentially marine, 

 include species characteristic of an estuarine environment. As examples of these we 

 may mention Nassa and Thais among the Gastropods and, among the Lamellibranchs, 

 Modiola, Area, Meretrix, Corbula, Martesia, dementia and Theora. The species of 

 Pholadidae, Teredinidae, Arcidae and Solenidae are, however, quite distinct from 

 those that have established themselves in the Ganges and other Indian rivers. 



GEOGRAPHICAL DISTRIBUTION. 



With the exception of Chilkaïa, all the genera that comprise the molluscan fauna 

 of the Chilka Lake have a very wide geographical distribution, whereas, as we have 

 already pointed out, more than one third of the species at present appear to be 

 endemic. Apart from apparently endemic species the Mollusca of the lake fall, with 

 one or two possible exceptions, into two categories, (a) those that are found only in other 

 localities of a similar nature on the Indian coasts and (b) those of wide distribution. 

 The number of the former is comparatively small, but with further exploration it is 

 probable that many of the apparently endemic species will be transferred to this 

 category. The following forms are known to occur both in the Chilka Lake and in 

 estuarine tracts in other parts of India, but have not been found elsewhere : — 



Gastropoda. 



Tomatina estriata. 



Vanesiarambhaënsis. Lameixibranchiata. 



Nassa denegabilis. 



... . dementia annandalei. 



,, onssaensis. 

 tj i 7- ,„, ,. ,. Cusétdaria annandalei . 



Hydrobia (Belgrandia) myhacea. 



Stenothyra blanfordiana. 



A considerable amount of work has been done by Nevill, W. T. Blanford, Ben- 

 son, Stoliczka, von Martens and Preston on the aquatic shells of estuarine tracts in 

 India and the Malay Archipelago; but (except in the case of the-last author) most of 

 their papers refer exclusively to species found at the edges of creeks and backwaters 

 or in small pools of brackish water. This is probably one of the reasons why our 

 collection from the Chilka Lake differs very greatly from those previously described 

 from similar localities, a very large proportion of the species having been obtained 

 by dredging. A real difference, namely the scarcity in the lake of certain thick- 

 shelled amphibious forms, such as Neritina, Littorina and Pythia, is probably ex- 

 plained by the absence of mangroves and semi-aquatic palms to the stems of 

 which such species frequently attach themselves. It is less easy to explain the entire 

 absence of the almost terrestrial mud-loving genas Onchidium and the absence or 

 scarcity of the aquatic genera Iravadia and Corbula, which are remarkably abundant 

 in the Gangetic delta. The occurrence of Cyclostrema is, however, interesting, as we 

 believe that Nevill's "Valvata? microscopical a species very abundant at Port 

 Canning, also belongs to this genus. 



Some years ago a considerable collection of shells was made in shallow water off 



