1916.] Fauna of the Chilka Lake : Mollusca Gastropoda , etc. 337 



Two species of molluscs were found only in or on wooden posts set up to mark 

 the fairway in the outer channel near Satpara. These were the ship-worm Xylotrya 

 stutchburyi and an oyster (Ostrea sp.) of which a few large individuals were obtained 

 but have unfortunately been mislaid. 



The periodic growth and decay of the thickets of weed to which we have alluded 

 above is an important factor in the distribution of the Lamellibranchs Modiola undu- 

 lata and Cuspidaria annandalei. The former is known to breed in the lake at all 

 seasons and is found on filamentous algae growing on stones, but by far the greater 

 number of the individuals observed were attached to thicket-forming weeds. Almost 

 as soon as these weeds begin to grow up they are covered with young mussels, 

 which increase in size rapidly and evidently become mature before the plant dies 

 down. The same fact was noted to a less extent in the case of Cuspidaria. It is 

 of importance to the fisheries of the lake, in that several of the more abundant edible 

 species of fish haunt the thickets and devour weeds and molluscs together. 



Several Gastropods also frequent weeds, notably the species of Stenothyra, Litio- 

 pa, Pyrgulina and Chrysallida ; but these are also found in large numbers among 

 algae of less luxuriant growth and do not form so characteristic a feature of the 

 thickets. Nassa denegabilis and N. orissaënsis apparently crawl indifferently among 

 weeds or on bare mud. 



The great majority of the Mollusca found in the lake inhabit it throughout the 

 year: but it was observed in the case of several of the commonest species, e.g. Toma- 

 tina estriata, dementia annandalei and Theora opalina, that a very large number of 

 individuals died in the latter part of the freshwater season— a fact of particular 

 interest in view of the marine origin of the fauna. It would seem that in the Mol- 

 lusca, as also in other groups, certain individuals are more tolerant of changes in 

 salinity than the majority of their kind, and that the effect of fresh water on the 

 organism, in at least some forms, is cumulative rather than suddenly fatal. The 

 small Opisthobranch Bulla crocata affords interesting evidence. It was originally 

 described from sheltered positions in the sea and is not uncommon, at any rate in 

 certain seasons, in the Madras backwaters. The only living specimen we found in 

 the Chilka Lake was taken in fresh water (in September, 1914), but was scarcely half 

 the normal size, though the shell was fully formed. Full-sized specimens that had 

 not long been dead were obtained, in the same month and in the same part of the 

 lake, among decaying weed cast up on the shore. It would seem probable that the 

 species makes its way into the lake either in the larval stage or before its growth is 

 completed and that the majority of the individuals which have attained their full size 

 in the salt-water season succumb to the freshwater floods. An unusually hardy indi- 

 vidual, however, occasionally survives throughout the year, but is dwarfed by the 

 unfavourable character of its environment. 



A number of other species are represented in our collection only by fresh but 

 empty shells, found in the outer channel in September in circumstances that did not 

 suggest their having been introduced artificially. As examples we may mention 

 Cyclostrema (Tubiola) innocens and Epitonium hamatulae. In several cases, notably 



