I92I.] N. Annandale & R. B. S. SeweLL : Vivipara. 261 



prominent) in the full-grown shell. Three primary rows of perios- 

 tracal chaetae can be distinguished, the best developed of which 

 runs round the periphery of the whorls, while the other two are 

 situated above it. The peripheral row, though the most impor- 

 tant of the three, is the latest to be formed and only the first or 

 uppermost row extends to the apex of the protoconch. In some 

 species (e.g. Vivipara dissimilis) , two additional rows of chaetae 

 are present on the embryonic shell, one between the peripheral 

 ;uid middle row and one above the first row. These chaetae are, 

 however, smaller than those of the three primary rows. They are 

 homologous with two of the fine spiral ridges on the shells of other 

 species. 



In those species in which the shells are ornamented with 

 bands of dark pigment the colour-pattern is periostracal in origin, 

 though the calcareous matter may be slightly stained. The bands 

 correspond in position with the rows of chaetae and spiral ridges. 



The test-sculpture {i.e. that of the outer calcareous la3'ers of 

 the shell) also corresponds in position with that of the periostracum. 

 In shells of the family in which it is highh' developed, it consists 

 mainly of orominent spiral ridges. These ridges may be smooth 

 and uninterrupted (as in Lecvthoconcha lecylhis) or broken up more 

 or less distinctly into series of tubercles (as in some individuals of 

 Taia naticoides) , scale-like projections (as in the most highly deve- 

 loped shells of Taia and Margarva) or even spines, as in the fossil 

 Rivularioidcs. ' They may be hollow as in V . oxytropis or solid as 

 in the Chinese V . lapiUorum. In practically all shells of all types 

 the most prominent and most highly developed ridge corresponds 

 with the peripheral row of chaetae , and in a large proportion those 

 that correspond with the two other primary- rows of chaetae are 

 better developed than any others. Moreover, each ridge corres- 

 ponds either with one of the primary rows of chaetae or with a 

 secondary ridge of the periostracum. 



Summary Account of the Structure of the Edge 

 OF the Mantle. 



We may summarize the structure of the distal part of the 

 mantle in the Viviparidae as follows : — 



The free edge of the mantle is membranous, but much thicker 

 in some genera (e.g. Lecythoconcha) than in others. The margin 

 bears at least three digitiform processes, which are better deve- 

 loped in some species (e.g. Vivipara oxytropis) than in others, and 

 are usually obscured bj- contraction and shrinkage in preserved 

 material. In addition to those three primary processes other, 

 smaller processes are present, probably in all cases, but are still 

 more difficult to detect except in the living animal and may per- 

 haps become vestigial in the adult of certain species. These pri- 

 mary and secondary processes correspond in position with the 



' Annandale, Rec. Geol. Surv. Ind. I., pi. xxxiii, fig-<i. 7-12 (igig). 



