1918.] N. Annandale: Molluscs of the Inle Lake. 169 



hinges of the shell, and these variations seem to be entirely individual. 

 The Yawnghwe river-species differs from the lake-form in its smaller, 

 thicker and higher shell and also in the very feeble development of 

 the wing. Possibly this last character is correlated with life in much 

 stiffer mud. This species is not a variable one. 



The only Corhicida found in the Inle basin is C. noetlingi, a form 

 that has a wide range in the Shan States. Von Martens refers to a 

 small variety which has a certain difference in outline from the typical 

 form, but we did not find this variety in the Inle and He-Ho basins. 

 The species lives in streams and does not enter the lake. Subfossil 

 shells do not differ constantly from living ones and there is no very 

 definite racial difference between shells from altitudes varying from 

 3,000 to 4,500 feet, but the local phase has probably developed into a 

 distinct race. I have refrained from giving it a name and from 

 discussing it in detail because great confusion exists as to both the 

 nomenclature and the specific limits of the eastern Corbiculae. 



Pisidium casertanum is perhaps the most interesting bivalve mollusc 

 found in the district, as it is a characteristic Palaearctic form. Speci- 

 mens from the lake do not exhibit any great individual variation. The 

 species is an extraordinarily plastic one, with a very wide geographical 

 range. The limits of its variation have been discussed in great detail 

 by B. B. Woodward^ in his catalogue of the British species of the genus. 

 He says : — 



" In external conformation this is a most variable species and maj' at times, especially 

 when dwarfed, resemble forms of P. pnsiUiim .... P. ijersonatum . . . . and even 

 P. nitidum. 



" Ihere is one well marked form, a lake or still water form, which almost amounts 



to a variety. In this the shell is rounder than the type, and more compressed 



whilst the hinge being narrower and lighter is less arcuate and the Hexure less pro- 

 nounced." 



In three localities in Eastern Asia a form has been found that re- 

 sembles " this lake or still water form," viz., in Lake Biwa in Japan, 

 in Lake Baikal in Siberia, and in the Inle Lake on the Shan Plateau. 

 I have not seen shells from Lake Baikal, but those from the Inle Lake 

 resemble Lindholm's figure ^ very closely. The Japanese shell, which is 

 a little wider than Inle specimens, has been examined by Woodward,^ 

 who remarks that it was " rather more oval than usual," i.e., in the still 

 water form. A single shell from a stream on the He-Ho plain is much 

 more inflated than those from the lakes and approaches the Himalayan 

 P. atkinsonianum,'^ which is also found in streams. 



CONCLUSIONS. 



This is not a general treatise on variation or evolution, but merely an 

 attempt to demonstrate so far as demonstration is possible, and with 

 as little reference to contentious works as may be, certain phenomena 



^ Catalogue of the British Species of Pisidium in the Collection of the British Museum, 

 pp. 31-44, pis. i, figs. 3-0 ; iii, fig. 3 ; xiii-xviii (London : 1913). 



2 Lindholra in Korotneff's Wiss. Ergebn. Zool. Exp. Baikal-See, IV (Moll.), p. 85, pi. 

 ii, figs. 4.5, 46. 



3 See Preston, Ann. Mag. Nat. Hist. (8), XVII, p. 162 (1916). 

 « See Preston, Faun. Brit. Ind. Moll, p. 226, fig. 29 (1915). 



p2 



