rgar.] N. Annandale & R. B. S. SewEll : Vivipara. 221 



mantle, where it joins the sides of the body, is thickened owing to 

 wing-like expansions of the body muscle which pass outwards and up- 

 wards within its substance. Posteriorly the edge of the mantle is 

 continued round the lower aspect of the bodj'; l)etween it and the 

 upper surface of the foot, as a narrow pigmented ridge. Running 

 upwards in the floor of the branchial chamber is the branchial 

 fold. This structure presents a somewhat different appearance in 

 the two sexes. In the female it forms a thin crenated fold, which 

 passes upwards to the extreme apex of the branchial cavity, and 

 finally becomes closely connected with the upper end of the gill 

 immediately in front of the pericardium. Below, it is continuous 

 «ith the left fold of the syphon and the ridge passing forwards 

 beneath the right tentacle on the right side of the snout. Through- 

 out its whole length it bears on its right side a small sub.sidiary 

 ridge or fold, which is usuallj' of a brown colour and which 

 lodges a blood sinus. In the male the ridge consists of a stout 

 basal portion, which is surmounted bj' a thin lamella. The reason for 

 this difference in the two sexes lies in the fact that in the cf this 

 ridge accommodates throughout almost its entire length the vesicula 

 seminalis. According to Moore (1901, p. 470, note to fig. i, pi. 

 xxv) in the closely-related mollusc, Neothauiiia tanganyikense. 

 Smith, this ridge serves as a protection for the gill against damage 

 from pressure against the distended uterus and contained j'oung, 

 and is better developed than in Vivipara vivipara. It is always a 

 matter of some difficulty to compare living examples of one species 

 with the published descriptions and figures of others, but it seems 

 to me that in Vivipara bengalensis this branchial fold is every whit 

 as well developed as in Neothaiima tanganyikense, and a study of 

 the living animal has convinced me that Moore is wrong in his 

 view of its function. If a fully-expanded example of V. bengalensis 

 be examined in the live state, it will be seen that the branchial 

 fold extends vertically, inclining slightly to the right from the 

 floor of the branchial chamber till its upper free border almost if 

 not actually reaches the roof of the chamber, thus dividing the 

 branchial chamber into two almost completely separate parts. 

 On the left of this ridge is a wide cavity tlie upper and left wall of 

 which is formed by the gill ; and a little behind the mouth of the 

 shell, the tips of the gill-filaments are in close apposition to the 

 free border of the fold. A transverse section about half way up 

 the body-whorl shows that the tips of the gill-filaments may 

 actually pass across above the upper edge of the fold and project 

 into the cavity on its right side. The cavity to the right of the 

 branchial fold is never completely occluded by the uterus ; there 

 is always a free interval between the two which becomes con- 

 tinuous below with the syphon tube. The ridge is covered with a 

 tall columnar ciliated epithelium, very similar to that covering 

 the gill lamellae and the margin of the mantle. The presence of an 

 equally well-developed branchial fold in the male indicates that 

 Moore's explanation is not the true one and I entirely agree with 

 Cuvier (181 7, p. 6) that its function is respiratory, though it is 



