I JO Records of the ludian Museum. [Voi,. XIII, 



smaller lime content in the west coast rivers. Here I should men- 

 tion that much of the variation found in the form and proportions of 

 the umbonesand hinge region of M . casta, is due to the effect upon 

 the size and relative relationship of these parts, caused by the ex- 

 ceedingly variable rates at which the shell increases in thickness 

 under different conditions. Exceptionally rapid growth in thick- 

 ness tends to produce a humped and corbicular shape, while rapid 

 growth in length and breadth with slow deposits of lime salts give 

 a compressed form with flat umbonar region. Under certain con- 

 ditions not now prevailing, but which existed at the very recent 

 geological period when the shell-pits of swamps round the margins 

 of the larger backwaters of the eastern coast were being formed, 

 the deposit of lime salts must have progressed at a greater rate 

 than the most rapid now existing,^ for in these sub-fossil deposits 

 we got an immensely massive form of M. casta. This shell at first 

 sight appears so different in hinge form and in general shape from 

 the type of M. casta that one does not hesitate to treat it as a dif- 

 ferent species. Hence we find Preston describing it as a Cyrenid 

 under the name of Corbicula {Velorita) satparaensis. He saw cause 

 to modify this opinion later and in Rec. Jnd. Mus., XI, p. 300, he 

 rightly assigned it to M. casta. In this I agree with him, after a 

 comparison of a long series of young individuals of the massive 

 form as opposed to those of the type. This gave conclusive 

 evidence of identity, as the extreme comparative solidity of the 

 variety is rapidly lost as we descend in the series till at last 

 among small individuals of the type and of the variety, of f inch 

 in length, we attain practical identity and it becomes impossible to 

 differentiate the one from the other. 



Another factor which has a determining influence in modify- 

 ing the form of the shell is current action. In channels and creeks 

 where the current is slow and weak, the inflated cordate and ovate 

 forms persist ; where the current is strong, the form tends to become 

 abnormally elongate and flattened. Such condition and effect are 

 seen wherever M. casta has managed to establish itself in the main 

 channel of the west coast rivers at a relatively considerable dis- 

 tance from the sea. Elongation is particularly strong in case of 

 young individuals. This change is an adaptation to counteract 

 the danger which a rotund form such as that of the type would be 

 subject to when exposed to strong current influence, The flat- 

 tened form is less liable to be rolled along, just as this same general 

 shape has similar utility in the case of Donax cuneata which lives 

 in the surf-troubled sands of our beaches, where a rounded form 

 would subject it to the peril of being rolled forwards and back- 

 wards on the beach with every alternate surfbreak and backwash. 



Distribution of M. casta {type). — East coast of India, from the 

 Chilka Lake to Tuticorin in backwaters and connecting canals. 

 Also Ceylon and Singapore according to Romer ; it certainly 

 occurs in Ceylon, but I am very doubtful in regard to Singapore 

 as all the specimens in the Indian Museum cqllection from this 

 locality undoubtedly belong to var. ovum. 



