1 92 1.] Manipur Molluscs. 627 



headquarters in China, but it has colonized the Manipur Valley 

 successfully and is at home in practically every part of its waters 

 except in streams and rivers. Its plasticitv is remarkable, and 

 has probably aided it in taking possession of a very large territory. 

 In the Manipur Valley we found no less than four phases common, 

 each in its proper environment, and, so far as I know, only one of 

 these phases has been found outside the valley, unless the locality 

 " Sylhet " is correct for the forma typica, which I doubt greatly. 

 Should my doubt prove unfounded it will not alter ray argument. 

 Of the four phases the largest and best developed is the one found 

 in the central parts of the Loktak Lake, amidst dense submerged 

 vegetation but in comparatively clean water of relatively con- 

 siderable depth. The shell in this phase provides less evidence of 

 interrupted growth than any of the others, less individual varia- 

 tion and as a rule a greater symmetry in proportions. It is, indeed, 

 of just such a type as might be expected to occur in conditions in 

 every respect favourable to the species. The only approximation, 

 however, to a true lacustrine type exhibited b}^ it is its comparative 

 thinness. It has no tendency whatever to assume the elongate 

 conical outlhie of the lacustrine species of Taia. Indeed, it is 

 more globose than the shell of either the phase found at the edge 

 of the great swamp or that found in ponds. The rice-field phase, 

 on the other hand, is still more globose than the deep-water one, 

 but does not possess its symmetry or constancy to type. 



It is evident that we are here dealing with plasticity of a 

 somewhat different type from that illustrated by the genus Taia 

 in the Shan States, and with one in which the direct result of 

 environment on the individual may be more safely postulated. 



Indirectly the structure and post-embryonic development of 

 L. lecythis cast an interesting sidelight, though the adult shell is 

 smooth or nearly so, on the question of the development of promi- 

 nent spiral sculpture on the shells of the Viviparidae in certain 

 circumstances, but this point can be discussed more clearly after 

 the facts about Vivipara oxytropis have been summarized. 



Of the three species of Vivipara found in Manipur two are 

 verj' scarce and have not been seen by me in their natural surround- 

 ings. The third (F. oxytropis) is, however, abundant and shares 

 with L. lecythis the position of a dominant species throughout the 

 valley. Two points have to be considered in reference to this spe- 

 cies, its plasticitj' and its peculiar sculpture, the latter not so much 

 for its own sake as for the light it throws, taken with certain facts 

 in the life-history of L. lecythis, on larger questions. 



V . oxytropis is not quite so abundant or so universally dis- 

 tributed in the IManipur valley as L. lecythis. It is very nearly if 

 not quite as common in the Loktak Lake, but much scarcer in most 

 ponds and practically absent from the smaller swamps. This may 

 perhaps be correlated with two facts, firstly that it is not nearly so 

 plastic (i e. cannot adapt its external form to different types of 

 environment so well), and secondh' that it is so largely parasitized 

 not only by a trematode {Lencochlaridiiim enc3^sted in its mantle, 



