154 Recnrdft of the Jndian Museum. [Vol. XIV, 



Eoszkowski reared in captivity eggs of L. profunda, which he regards 

 as synonymous with L. foreli, from deep water. He found that after nine 

 months the shells of the young molluscs showed an extraordinary 

 resemblance to those of the shallow-water form L. palustris, Miiller. 



The only other genus of true aquatic pvdmonates represented in the 

 Inle basin is Planorbis, the shell of which is apparently less plastic 

 than that of Limnaea. Possibly the flattened spiral form offers less 

 opportunity for variation. Be that as it may, the species of the genus 

 that occurs in the Inle Lake fall into two categories. We have on the 

 one hand the widely distributed, comparatively large and thick-shelled 

 species P. exustus (belonging to the group or subgenus Planorbis, s.s.) 

 the geographical range of which is very great and the variation and 

 plasticity small. On the other hand we have a number of minute thin- 

 shelled forms of the groups or sub-genera Gyraulus and Segmentina, most 

 of which have also a wide distribution, while their variation, though 

 by no means extreme, is in some respects less restricted. P. exustus 

 need not concern us further in this connection as it is in no sense a 

 variable form. The smaller species (P. calathus, P. caenosus, P. velifer, 

 and P. trochoideus) are not, however, altogether devoid of interest. 

 With the exception of P. velifer, which is only known as yet from the 

 Inle Lake, all these species have in the lake even smaller and thinner 

 shells than they do in other localities. Their shells, moreover, are al- 

 most or completely devoid of colouring matter, while the pigmentation 

 of their soft parts is also reduced. We have a few specimens of P. 

 trochoideus from a lacustrine deposit in the He-Ho plain ; so far as can 

 be ascertained, they did not differ when living from living individuals 

 in the existing lake. 



P. velifer is interesting for two reasons. In the first place it is little 

 more than a highly specialized, dwarfed race of a widely distributed 

 form (P. convexiusculus), which in its turn is possibly no more than a 

 variety of P. compressus or saigonensis, a species of still wider distri- 

 bution. Shells intermediate between P. convexiusculus and P. saigon- 

 ensis occur in the superficial deposits of the He-Ho plain and only differ 

 from the typical Indian form of the latter species in being rather smaller 

 and in having the peculiar structure of the aperture that distinguish 

 the form from P. saigonensis, a little less strongly developed. From 

 these shells those of living individuals of P. velifer differ in their smaller 

 size, still less developed aperture, thinness and transparency. In- 

 dividuals from the middle of the lake are absolutely colourless, while 

 those from the margin have a faint yellowish tint. 



There is very little variation in the shape of these living shells, but 

 their ornamentation is by no means constant. In some examples there 

 are a number of curious spiral ridges on both surfaces of the shell. 

 Examined under a fairly high power of the microscope these ridges are 

 seen to be entirely epidermal, and to consist of closely adpressed cilia 

 or minute horny processes. The rows of these processes vary in number 

 and degree of development ; sometimes they are quite absent, though 

 at least ti aces can usually be detected by careful examination. Varia- 

 tion in this respect is entirely individual and occurs both among the 

 colourless shells from the centre of the lake and among the tinted shells 



