244 ■ Records^ of the Indian Museum. [Vol. XXII, 



In considering the ornamentation of the shell both colour-pattern 

 and sculpture can, therefore, be taken into account. 



Ornamentation of the embryonic shell. 



The ornamentation of the embr^^onic shell is almost uniform 

 in pattern in all species of Viviparidae investigated, and the only 

 important differences found are those in the degree to which the 

 spiral sculpture is developed in different forms. Colour-pattern is 

 usually absent, the shell being of a pale hornj- j-ellow or brown, 

 with the protoconch darker and browner than the rest ; but in 

 those species in which the embryo has a comparatively large num- 

 her of whorls before birth the dark spiral bands characteristic of 

 some such forms begin to appear on the younger parts of the shell 

 before it is set free from the egg-membrane. 



The sculpture at this early stage is mainly periostracal, in- 

 volving onl}' the horny outer covering of the shell ; lnut as this is 

 not entirely so I propose to discuss the periostracal sculpture first 

 and then that of the calcareous part of the shell or true test. It 

 will be convenient to treat V. bengalensis as a typical form in dis- 

 cussing the periostracal sculpture of the embryo. 



The shell of this species consists at birth of 3J whorls. Of 

 these the apical whorl and a half constitute the true protoconch. 

 They are flat, band-hke and almost smooth, but with a strongly 

 marked keel running round the outer edge of their upper surface in a 

 spiral. Several faint, line-like spiral ridges, of which two are more 

 prominent than the others, can also be detected on their surface 

 under a high magniiication. A single spiral row of extremely fine 

 hair-like processes projects from the marginal keel, extending up- 

 wards to the tip of the apical half-whcrl. Towards the base of the 

 protoconch these processes become stiffer and are curved and 

 retroverted at the extremity, the curvature of their tips being 

 directed towards the mouth of the shell. They also become less 

 crowded Together. A little above the poinL at which the pro- 

 toconch merges into the uppermost whorl of the younger part 

 of the embryonic shell {i.e. that part in which the whorls begin to 

 assume the essential characters of those of the adult), a second 

 line of chaetae makes its appearance parallel to the first, and finally, 

 on the penultimate embryonic whorl, a third. The oldest row, 

 which I shall call in reference to its age and its position the first 

 or UPPERMOST ROW, is rather less developed than the second or 

 MIDDLE ROW. The THIRD or PERIPHERAL ROW, which continues to 

 occupy the extreme periphery of the shell, is the largest and best 

 developed of all. As the shell grows, however, and new whorls 

 are added they destroy the chaetae that lie immediately above 

 them by the pressure of their embrace. 



Between the three primary rows of chaetae, above them to the 

 left of the mouth of the shell, and j^articularly below them to 

 the right (that is to say below the peripheral angle) there are other 

 spiral Unes on the external surface of the shell, running parallel 



