l74 Records of the Indian Museum. [Vol. XlV, 



be considered to be a highly modified form. The Succinea is a normal 

 form of its genus. 



The smaller moUusca of the Inle Lake, especially those belonging 

 to the genera Limnaea, Planorbis and Pisidium, would undoubtedly 

 be taken for deep-water forms by a conchologist accustomed to the 

 deep-water molluscs of the Swiss Lakes. ^ Their minuteness, their 

 fragility, the lack of pigment in both shell and soft parts, and, 

 in the Limnaea, the extremely long mouth of the shell and the rudimen- 

 tary but relatively narrow shape of the spire, are all features characteristic 

 of deep water. The Inle Lake was once very much deeper than it is now 

 and Limnaea mimetica is probably a deep-water form that has survived 

 from the period when that was the case, but two of the three species of 

 Planorbis that are now found in the central region of the lake are also 

 found, with some of the same characters, in waters that can never have 

 been deep and never even have formed part of a lake-system. The 

 Pisidium is no smaller or more fragile than the form (P. atkinso- 

 nianum) common in small streams in the Himalayas. Lideed, it is 

 very like this form, except that the shell is less swollen, a feature in 

 which it also differs from the phase found in streams on the He-Ho 

 plain. 



Moreover, smallness, fragility and colourlessness of shell in the case 

 of bivalve molluscs are often associated with life in soft mud in shallow 

 water in very different biological conditions. This is so not only 

 with P. atkinsonianum in small mountain streams, but also with 

 Corhicula tenuistriata ^ in the Whangpoo Kiver near Shanghai, and with 

 species of a number of different families in the brackish water of the 

 Chilka Lake.^ 



Li the life of these bivalve molluscs there is a common biological 

 feature in that they live in soft mud, and the modification of the shell may 

 be of practical utility to the individual and the race. In the bionomics of 

 Taia, Margarya and the Slavonian Viviparidae also it is not improbable 

 there is or was some common feature, but there is no evidence that it 

 was of biological importance and, if it existed, it is, with our present 

 knowledge, obscure. In any case the circumstances of their life-history 

 cannot have been by any means identical. All that can be said is that 

 in each case the pecuUar forms with highly sculptured shells seem to 

 have been evolved in a region of great lakes, in which an abundance of 

 soluble mineral salts was present and in which the climate was temperate 

 rather than tropical, warm rather than cold. Apparently also the water 

 did not possess any marked power of erosion of the shell, which would 

 have destroyed the sculpture and rendered its perfection impossible. 



From the facts stated and the inferences already drawn it seems 

 very doubtful whether the peculiar modifications of the shell observed 

 in so many of the aquatic molluscs of the Inle Lake can have any 

 bearing on the more highly specialized modern theories of evolution, 

 which, even if sound in certain instances, arc perhaps of less general 

 application than their rival exponents are willing to admit. None of 



^ See Zschokke, Die Tiefseefauna der Seen MiUelenropas, pp. 155, 164 (1911). 

 2 Annandale, Mem. As. Soc. Bengal, VI, pt. i, p. 07 (IDK)). 

 ' Annandale and Kemp; Mem. Ind. Mus., V, p. 341 (191(5). 



