628 Records of the Indian Museum. [Vol. XXII, 



as is also L. key this) but also by a leech of the genus Glossosiphonia 

 (against which the Lceylhoeoncha has a special protection, p. 549) 

 that it is probablj' able to survive only in favourable circumstances. 

 Moreover, we may correlate with these phenomena also the fact 

 that the species has a very liaiited range, not having been found 

 outside the valley except in one swamp in Tenasserim. That it is a 

 highly specialized form there can be no doubt. The main features 

 in which it differs from the majority of its congeners and of the 

 species of Lecythoconcha are the uninterrupted conical outline of 

 its shell, the prominent but hollow spiral ridges on the shell and 

 the great relative length of the processes on the edge of its 

 mantle. Its large size is also a characteristic feature. 



The conical outline of the shell is a specific, or rather group 

 character, not subject to marked individual variation or to 

 plasticity. V . oxytropis shares it with the much smaller and less 

 highly specialized V. microchaetophora from the plains of Eastern 

 Assam. The spiral ridges on the shell are evidence of higher 

 specialization and are not shared with V. microchaetophora ■ but 

 they are remarkably constant in the species and are certainly 

 correlated wiih the third anatomical character already mentioned. 



The processes on the edge of the mantle, though exception- 

 ally well developed in V . oxytropis, are not peculiar to that species, 

 but are found, in a less highly developed or rather more degenerate 

 condition, in V . bengalensis , in which they correspond in position 

 with the dark spiral bands on the shell just as they do with the 

 prominent ridges, which are also deeply pigmented, in V. oxytropis. 

 Moreover, similar p:ocesscs are present in young individuals even 

 of smooth-shelled species such as L. lecythis and then correspond 

 with spiral rows of chaetae on the shell which disappear as matur- 

 ity is attained. In the young mollusc, whether ot L. lecythis or 

 of V . oxytropis, there are three such processes, but whereas they 

 disappear altogether in the adult of the former species, they be- 

 come more numerous both in that of V . oxytropis and of V. ben- 

 galensis. In the adult V. bengalensis they are quite short even 

 when fulh' expanded and project from the edge of the mantle, 

 but in V . oxytropis they are much longer and are bent back into 

 the grooves on the internal surlace of the shell that corresponded 

 with the raised ridges on the external surface. The primar}^ 

 reason for their hypertrophy is probably, as I have pointed out 

 on p. 549, that they function as an accessory breathing organ. 

 The lidges on the shell in which they are lodged serve to protect 

 them and have thus a definite use, unlike the sculpture on the 

 shells of Taia or Margarya. 



As these processes and ridges on the surface of the shell of 

 V . oxytropis are constant they have little direct reference to either 

 variability or plasticity. Indeed, the species is neither remarkably 

 variable nor remarkably plastic. Male and female shells differ 

 somewhat in outline, and individuals from ponds vary more, have 

 not quite the same regularity of outline and do not as a rule grow 

 so large as those from the Loktak Lake, but no more can be said. 



