I92I.] Manipur Molluscs. 629 



The importance of V. oxytropis in the study of these phenomena 

 only becomes apparent when we compare the structure of its 

 mantle and shell with those of the mantle and shell of Taia and 

 contrast the constant character of the Manipur species with the 

 l)lasticity and variability of such a species as 7". naiicoides. This 

 I have done in another paper ' in the Records of the Indian 

 Museum. 



We may now turn to the Limnaeidae of the Inle and Lok- 

 tak Lakes. In the former body of water three species have been 

 found, namely Uninaea shanensis, Annandale, L. andersoniana, 

 Nevill and L mimeiica, Annandale. The last is a small and 

 highly peculiar species only known from the Inle Lake and not 

 exhibiting noteworthy variability or plasticity, except in so far 

 that it is probably as a species the product of plasticity in some 

 form of the L. aciuuinata group. L. shanensis is not. strictly speak- 

 ing, a variable species, and we only know that it is or has been 

 highly plastic through the existence of fossil or subfossil phases. 

 With L. andersoniana I will deal presently. 



In the Loktak Lake the onl3' species of Limnaea collected 

 was L. acuminata , but we may consider with it two other species 

 found in swamps or ponds in the Manipur Valley. These are 

 L. ovalior. sp. nov., and L. andersoniana, Nevill. 



L. acuminata provides us with one of the best examples of 

 true or individual variability to be found in the genus. In 

 some districts (see fig. 12, p. 569) there is a very great difference 

 in the shape of different shells from the same environment, but 

 this is not so, apart from aberrations or monstrosities, in the Lok- 

 tak Lake. A slight plasticity, however, is to be found in that 

 individuals from the less congested parts of the swamp have a 

 distinctly smaller shell and a shorter spire than those from the 

 margin, while those from a small sluggish stream in the vicinity 

 have remarkably pale and fragile shells with a strong but irregular 

 external sculpture. 



An interesting aberration is represented in our collection by 

 a single specimen. It is remarkable for the very poor develop- 

 ment of its spire, a feature common in lacustrine forms of the 

 genus. 



L. ovalior is known only from the swamps that surround the 

 Loktak Lake and from Dimapur in the plains of Assam, north of 

 the Naga Hills. In the latter locality it was found in a single 

 pool of very foul water. .Shells from this situation differ from 

 those from the Manipur swamps in the same way as, but to a 

 greater extent than, those of L. acuminata from the more con- 

 gested part of the Loktak Lake do from those of the same species 

 from its open region. 



It is in L. andersoniana, however, that plasticity occurs in 

 the most highly developed state. In the Inle Valley two forms of 



\ol. XXn. pp. 243-266 (1021;. 



