1918.] N. AnnAKdalE: Molluscs of the Inle Lake. 155 



from the marginal zone. Its explanation can hardly be the same as 

 that which would apply to the gradual change in the shape of the shell 

 illustrated by the various phases of Limnaea shanensis. In the one 

 case we are dealing with mere individual variability, on the other with 

 true plasticity. 



Prosobranchiata. 



Of the Pectinibranchiate genera that are found in the Inle basin 

 there is nothing particular to be said about A7mncola, the single 

 species of which seems to be very constant, while in Vivipara and 

 Ampullaria the material available so far as the Shan States are con- 

 cerned does not suffice for serious study. I shall have to notice a pecu- 

 liarity of certain shells of Aynfullaria, but will do so in discussing those 

 of Hydrohioides in the final part of this section of my paper. In Hydro- 

 hioides, however, in Melania, and above all in Taia the variation 

 observed is of a very remarkable character. 



The only species of Melania found in the Inle Lake is M. tuherculata. 

 The shell of this species, which has an extremely wide range in Africa 

 and Asia, is very plastic as regards its size and sculpture, though varia- 

 tion in shape is less common or at any rate less marked. In the Inle 

 Lake and also in the Yawnghwe river the shells (fig. 6a) are of a rather 

 small size, not exceeding 25 mm. in length. They are of dark colour and 

 have the tubercular sculpture characteristic of the species well-marked. 

 In a ridge of recent tufa on the He-Ho plain we found a number of 

 shells of much larger size, from 38 mm. to 40 mm. long (fig. 66). They 

 retain remains of the epidermis, which appears to have been dark, and 

 their sculpture is less strongly developed. On another ridge on the 

 same plain, however, we found shells (fig. ^c) not more than 16 mm. 

 long with the sculpture very strongly developed and with the suture 

 more impressed than usual. Unfortunately we have no information as 

 to the differences in environment correlated with this difference in 

 size and form of shell. 



At first sight it is remarkable that in this species shells from the 

 central region of the lake, where the water is extremely clear and where 

 vegetation is rather abundant, differ in no respect from those taken in 

 the muddy streams that traverse the Yawnghwe plain, but in both 

 types of environment the animal lives almost buried in very soft mud. 

 Moreover, there is evidence that the factor or factors which most 

 strongly influence plasticity in this species are not always those that 

 are most conspicuous or most readily ascertained. For the sake of 

 clarity in discussing the facts known to me I will confine my statement 

 on this species to forms that I have been able to investigate in the field 

 myself. I have collected specimens chiefly in four districts, viz., the 

 lake of Tiberias in Palestine, the Gangetic Delta, the shores of the 

 Chilka Lake on the east coast of India, and Yawnghwe State; The 

 normal form of the species, that is to say, the one that approximately 

 strikes the mean between extremes, chances also to be the forma 

 typica, which was described from the coast of Madras. In this form 

 (fig. 6a) the size is moderate, the length being from 25 to 30 mm., the 

 spire tapers gradually, the colour is brown or dull green with more 



