364 



Memoirs of the Indian Museum. 



[Vol. V, 



selves found in small pools near Calcutta liable to desiccation, none exceed 23 mm. in 

 length. There are several other series in the collection of the Indian Museum from 

 the Gangetic delta which include shells 31 mm. long, but we are ignorant of the 

 precise circumstances in which they were found. 



From all these facts it would seem that the small size of the mussels of this spe- 

 cies found in the Chilka Lake is in no sense a racial character, but is due to the 

 direct effect of environment on the individual. We must remember that by far the 

 greatest part of the rock- area available on the shores and islands of the lake is com- 

 pletely dried for several months in the year, at any rate from March until the latter 

 part of June. At the end of the dry season extremely few living individuals are to 

 be found and these are situated in close proximity to the muddy bottom and are 

 therefore liable to be buried. From the situation in which the young mussels 

 establish themselves it necessarily follows that the chief, though not the only, breed- 

 ing season must occur shortly before the adults are killed by the sinking of the water- 

 level and that the larvae settle down when the lake is full. It is interesting to notice 

 that they do so at a time when the water is quite fresh or but very slightly saline. 



The situation most favourable to the growth of M . striatula seems to necessitate 

 the following conditions, — (i) a firm support provided with cavities in which the 

 animals may attach themselves ; (ii) the absence of any risk of being engulphed in 

 mud or in living sponges and (iii) an uninterrupted supply of water. There is of 

 course the question of food-supply also, but on this we have no information. To 

 judge from the specimens we have examined, ideal conditions are to be found on 

 worm-eaten logs of wood, either fixed beneath the lowest water-level or floating. 



It is not improbable that the species is essentially an estuarine one, but in spite 

 of this fact, ideal conditions exist very rarely, if at all, in the Chilka Lake. We are 

 of the opinion that dwarfing in the case of M. striatula in the lake is not due to the 

 low salinity of the water and that there is no evidence that the unfavourable condi- 

 tions noted in the preceding paragraphs have affected the race as distinct from the 

 individual. 



M. striatula was originally described from the Philippines and has been recorded 

 from the Gulf of Siam, Singapore, Ceylon, Burma and from both sides of the Indian 

 Peninsula (Calcutta, Madras, Cochin, Bombay). 



BIBLIOGRAPHY OF INDIAN BRACKISH- WATER MOLLUSCA. 

 Benson, W. H. — Conchological notices ; chiefly relating to the land and fresh-water 

 shells of the Gangetic provinces of Hindoostan. — Zool. Journ., V, p. 458, 



i835. 

 Descriptive catalogue of a collection of land and fresh-water shells, chiefly 



contained in the Museum of the Asiatic Society. — Journ. As. Soc. Bengal, 



V, p. 741, 1836. 



Description of the shell and animal of Nematura, a new genus of Mollusca 



inhabiting situations subject to alternations of fresh and brackish water. — 



Journ. As. Soc. Bengal, V, p. 781, 1836. 



