i68 Records of the Indian Museum. [Vol. XIII, 



lower classes esteem it because of its cheapness and tastiness, and 

 its shells, owing to their solidity, have considerable value to the 

 local lime-burners as a source of lime. The inherent variability 

 already noted as of considerable range on the east coast is greatl}^ 

 intensi6ed on the west coast. This is due to two factors : (a) the 

 much more diversified conditions under which the species exists on 

 the latter coast, and (6) the vastly greater numbers of individuals 

 involved. 



On the west coast the rainy season is much more prolonged 

 and the rainfall much heavier than on the east ; as a consequence 

 the backwaters, even close to their seaward entrances, have a low 

 salinity for months in the year — during the height of the floods it 

 is difficult to conceive of the water in even the lowest reaches 

 being anything but wholly fresh Hence, to survive requires in 

 any estuarine mollusc marked power of adaptability to rapidly 

 altering physical conditions. This quality is considerable in M. 

 casta and is assisted by its numerical abundance, which furnishes 

 the requisite numbers for the successful intervention of natural 

 selection in the problem. The struggle for existence under these 

 conditions is obviously a hard one and the west coast races of M. 

 casta show the effects thereof in several characteristics. They 

 seldom exhibit vigorous growth ; dwarfing is the rule ; extreme 

 variations in form are to be counted as favoured if not induced by 

 these conditions. Malformations are numerous and corrosion of 

 the umbonar region is nearly always present. Complete adapta- 

 bility has not even yet been attained ; mortality is excessive 

 every flood season, and the vast majority of each generation do 

 not survive to a second year of existence. 



In the biological survey of such an estuary as that of the Balia- 

 patam river in North Malabar, it is most significant to notice that 

 only at the mouth are large and vigorous examples of M. casta 

 found strictly comparable with the type form inhabiting the sea- 

 ward channels of Chilka and Pulicat Lakes, where the duration of 

 river floods is short and where tidal influence is never wholly lost 

 except for a comparatively short period in each year. The most 

 seaward beds in the Baliapatam estuary ^ield shells almost exactly 

 similar to poorly grown east coast ones. In both, the ventri- 

 cose and cordate form is emphasized ; the colour of the Balia- 

 patam shells is stained rufous red as in Pulicat shells, and the 

 main distinctions to be noted in average shells are that the elonga- 

 tion of the posterior angle of the shell is slightly more pronounced, 

 while neither the escutcheon nor the vulva or coloured area of the 

 upper posterior region show more than a trace of the elevation or 

 reflection in the median line, which is usually present in most east 

 coast individuals. On the other hand, the differences between 

 selected specimens of the east and the west coast forms are less than 

 the differences between the extremes met with in purely east coast 

 or in purely west coast shells. 



In sheltered creeks where a fairly high salinity is maintained 

 for several months after spawning, individuals grow rapidly and 



