ig2r.] N. Annandai.e & B. Prashad : Iiidi in Molluxcs. 123 



included it in the family Hydrobiides. Preston ' followed Fischer 

 in keeping; Stenothyrinae as a subfamily of Paludestrinidae (^Hy- 

 drobiidae). We have already gi\en our reasons for following 

 Fischer. 



The position of the fossil shells assigned to the subfamily by 

 Fischer is doubtful. The Indian, Chinese and Malaysian species 

 we have examined belong to the single genus Steiwt/iyra , Benson, 

 which seems to be divisible into two subgenera, which we call 

 Stenotliyra (s.s.) and Asteuothyra nov. 



The subgenera may be distinguished on shell-characters' as 

 follows : — 



1. \'entral surface of body-whorl Hatteiied ; mouth \-er3' 



small, not at all prominent, separated from the outer 

 edge of the shell above by a uell-defined triangular 

 area ... ,. ... .... SfenotUyra- 



2. X'entral surface of body-whorl convex ; mouth larger 



and slightly prominent, less regular in outline ; 

 area separating it from the edge of the shell small 

 and ill-defined ... ... Astennt/iyra. 



Genus Stenothyra, Benson. 



The shell is small, rarely * exceeding 5 mm. in length, but 

 relatively' thick, ovate or subcylindrical, distinctly compressed in 

 the dorso-ventral axis, with at least 4 whorls, without prominent 

 sculpture. The umbilicus is closed or rimate. The aperture is 

 relativelv broad, ovate, oval or subcircular, oblique or transverse. 

 The peristome is continuous and uniform, never prominent and 

 barely thickened. The periostracum is well developed. 



The operculum is horny, but thick and containing much cal- 

 careous matter, very brittle, paucispiral on the external surface. 

 The internal surface is somewhat convex, smooth and polished 

 with a thickened rim. On it are developed two short, prominent 

 tranverse ridges, situated nearer the upper and lower extremities 

 than the centre. In the complete shell the operculum fits tightly 

 into the mouth, but in periods of active growth it is retracted as 

 far as the old peristome. 



The radula. — The central tooth is broad and has an enlarged 

 central cusp with much smaller lateral cusps on either side. It is 

 produced into a lateral process on either side and each process 

 bears a series of latero-basal denticulations. The outer teeth are 

 also relatively broad and have their denticulations rather poorly 

 developed. There is an enlarged central or internal cusp on the 

 inner lateral. 



' Preston, Faun. Brit. Ind. Freshw. Mull. p. 79 (1Q15). 



■^ In differentiating shells of this subfamily (and, indeed, those of all Hydro- 

 biidae) it is important that fully formed sjiecimens should be selected. Those 

 collected in periods of active growth have the peristome incomplete. The opercu- 

 lum, moreover, can be retracted as far as the old mouth and has its margin dis- 

 tinctly ciliate. 



■■* The shells of the Bornean species 6'. strigilata. Benson, measure as much as 

 8 mm. in length. 



