118 Records of the Indian Mvscum. [Vol. XIV, 



Nothinii is known of the operculum or of the soft parts, but the 

 shell is so like that of the species here described as Hydrobioides avarix 

 that they must be congeneric. 



The species was described from Kyoukpong on the Irrawaddi. 

 Several subfossil shells were found on the banks of the He-Ho stream. 



Hydrobioides nassa (Theobald). 

 Plate xiii, figs. 1 — 7 ; plate xiv, figs. 4, 4a. 



1865. Bithinia nassa, Theobald, Journ. As. Soc. Bengal, XXXIV (2), p. 275. 

 1870. Bithinia nassa, id., ibid., XXXIX (2), p. 402, pi. xviii, fig. 8. 

 1876. Bithynia nassa, Hanley and Theobald, op. cit., pi. xxxvii, figs. 8, 9. 



Theobald describes the shell as follows : — 



" Testa elongata, turbinata, polita, diaphana, solidusoula. Labio expansiusculo, 

 plica callosa externa munita. Anfractibus quinque. 0-45 0-25." 



I figure the radular teeth and external male organ on pi. xiv. Both 

 branches of the latter are well-developed and, from the point at which 

 the organ bifurcates to their tips, are about equal in length. The inner 

 branch is somewhat expanded distally, but flattened at the apex. The 

 outer branch tapers gradually to a point. The whole organ is practic- 

 ally smooth. A band of dark pigment runs along the middle of the 

 upper surface of the outer branch and of the basal undivided part. 



The expanded foot is tongue-shaped and truncate in front, with 

 the anterior angles slightly produced. The proboscis is moderately 

 stout ; not distinctly notched in front ; it extends beyond the anterior 

 margin of the foot. The antennae are filiform and when fully ex- 

 panded longer than the shell. They are, however, highly contractile 

 and one is often (temporarily) more extended than the other. The 

 foot, the base of the tentacles and the edge of the mantle are dull yellow ; 

 the proboscis is black with gold specks, becoming paler distally ; 

 the tentacles (except at the base, where they are pale) are greenish and 

 speckled. 



The operculum is narrowly ovoid, pointed and a little produced 

 posteriorly ; the marginal membraneous part is narrow ; there is a 

 well-marked depression in the centre of the external surface : although 

 the striae are concentric on the peripheral part of the surface, there 

 is a distinct spiral in this depressed region ; a low but broad ridge runs 

 along the internal margin. The whole operculum is whitish and trans- 

 parent. 



The shell, though not markedly variable in any one habitat, ex- 

 hibits considerable plasticity and it is possible to recognize at least 

 four races or phases among the living and fossil specimens we obtained. 



Theobald's types, of which there are several in the collection of 

 the Indian Museum, came from a locality situated at a considerable 

 distance north-east of the Inle basin. The form common in ponds 

 in that basin does not, however, differ in any important character 

 from these specimens. All that can be said is that the shells are as 

 a rule a little smaller and a little more conoidal. I think it best, there- 

 fore, not to distinguish this form by name and will refer to it as the 

 forma typica. The most interesting feature in which the four races 



