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



the marginal processes. This is proved not only by the position 

 of the bands and their arrangement on the shell but also 

 by the close correlation of dark pigment with the periostracal 

 glands in such species and by its absence from the margin in those 

 species the shells of which are not marked with dark spiral hands. 

 We may presume that, after the secretion of the horny fluid to 

 form the chaetae and ridges, the pigment is poured out in a 

 similar manner along the processes and deposited by them on the 

 surface of the lip. The chaetae themselves are not coloured by it, 

 and I do not think that the ridges are either, though this point is 

 difficult to observe with certainty, because the dark bands do not 

 appear until the shell has become fairly opaque and the ridges 

 project very little from the surface. 



To return to the periostracal sculpture, the line vertical 

 lines are evidently due to a pouring out of horny matter direct 

 from the supramarginal groove, with which they correspond 

 exactly in orientation, each representing, so to speak, a separate 

 act of secretion. 



The sculpture of the test also corresponds closely in position 

 and arrangement with that of the periostracum and, indeed, so 

 far as the minute sculpture is concerned, is practically a cast of it 

 from which the upright chaetae are necessarily omitted, just 

 as single upright hairs cannot be represented in a plaster cast. 

 In most forms of Vivipara bengalensis, and indeed in most Vivi- 

 paridae, there is nothing more to be said on this point, but in 

 those species which have highly sculptured shells, and even in 

 some phases and individuals of V. bengalensis, a further exposi- 

 tion is necessary. 



The highly sculptured shells among the Viviparidae fall into 

 three categories, viz. (i) very thin shells with uninterrupted, hollow 

 spiral ridges ; (2) thicker shells with uninterrupted, solid or 

 almost solid spiral ridges, and (3) shells, thick or thin, with 

 ridges that are more or less distinctly broken up into nodules, 

 scales or spines. 



Of the first type, which is the scarcest of the three, V. oxytro- 

 pis is an excellent example. The shell, in spite of its large size, is 

 exceptionally thin and fragile and the ridges upon it though promi- 

 nent are very little thicker than the intervening spaces on account 

 of the well-marked concavity of their internal surface, which forms 

 a regular groove. No peculiarity of the calciferous glands has 

 been observed in connection with these ridges on the shell, and 

 none is necessary to explain them, for thej^ lodge the greatly 

 hypertrophied marginal processes, the mere presence of which on 

 the internal surface of the shell while the calcareous matter was 

 soft is sufificient to account for their presence. 



On shells of the second type with solid uninterrupted ridges 

 no satisfactory direct observations have been possible, owing to 

 imperfect preservation of the material available, but the mar- 

 ginal processes are not hypertrophied as in V. oxytropis and it 

 may perhaps be assumed that the ridges are due to a certain 



