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



The chief reasons for the difference between the molluscan fauna of the two 

 regions appear to be two, — differences in salinity and differences in the nature of the 

 bottom, the latter factor being perhaps more important in the case of Mollusca than 

 in that of some other groups. 



Apart from these distinctions between the two regions, there are other divisions 

 in the fauna dependent on other causes : certain species are abundant in restricted 

 localities. Potamides fluviatilis , for example, is extremely common in very shallow 

 water on all ground in which the mud is mixed with sand, being apparently able to 

 endure a high temperature fatal to other species, but avoiding soft mud. The same 

 kind of bottom is also the only one that attracts the species of Lyonsia and Anatina, 

 but they are burrowing forms not so easily observed. 



The number of rock-haunting molluscs is very small; indeed, only two species, 

 Modiola striatula and Thais carinifera, can be assigned definitely to this category. 

 Both of these are abundant, but their distribution in the main area of the lake is not 

 the same ; for while the mussel is found in large numbers on all rocks that are sub- 

 merged for more than a few months in the year, the Thais is restricted to those south 

 of Kalidai, being found only in water the specific gravity of which never falls below 

 1-003. This species is also found on the oyster-beds at Manikpatna in salt water; its 

 distribution in the lake evidently depends not only on salinity, but to some extent on 

 the presence of mussels or other thin-shelled molluscs on which it preys. 



The oysters that occur in the Chilka Lake belong to at least three species, but 

 only one, Ostrea virginiana, is at all common Oyster-beds are found only in the 

 neighbourhood of Manikpatna. At this place several small sandy islands are so ar- 

 ranged as to form a bay sheltered from currents that would prevent the deposition of 

 spat, while the fact that the bay is situated at no great distance from the sea-mouth 

 is of importance, both because the greater part of the silt from the flood-waters has 

 already settled before the floods reach it, and because it obtains immediate benefit 

 from the irruption of sea-water that occurs when they subside. 



In March we occasionally found single living individuals of 0. virginiana 

 attached to rocks in the neighbourhood of Patsahanipur ; in some of them the shell 

 was as much as 3 cms. in diameter. Later in the year apparently fresh but empty 

 shells of a similar size were noted in the same place. We believe that this indicates 

 that a certain number of larvae make their way into the main area of the lake, on the 

 rocks of which, as we will show later, the oyster was once abundant. They are able 

 to settle down and to grow considerably, but are ultimately killed by the summer 

 floods. If this is so, the rate of growth must be very rapid, but the Uriya fishermen 

 state that when the oyster-beds at Manikpatna are overwhelmed with sand, as some- 

 times occurs in the flood-season, they are entirely renovated in a single year. The 

 bulk of the beds are formed of living and dead shells of 0. virginiana , to which a few 

 individuals of 0. cucullata and 0. lentiginosa, with large numbers of Modiola striatula , 

 attach themselves, while Petricola esculpturata esconces itself in cavities between 

 them. So far as we were able to observe, the last-named species was entirely free 

 from the necessity of constructing borings of its own. 



