XV. THE INDIAN MOLLUSCS OF THE 



ESTUARINE SUBFAMILY 



STENOTHYRINAE. 



By N. Annandale, D.Sc, F.A.S.B., Director and B. Prashad, 

 D.Sc , Offg. Superintendent, Zoological Survey of India. 



(Plate XVI). 



Among the estuarine Gastropods of the coasts of India, Ma- 

 laysia and China few are commoner and more characteristic, but 

 smaller and less conspicuous, than the Hydrobiidae of the sub- 

 family Stenothyrinae. These little water-snails, the shell of which 

 is rarely more than 5 mm. long, are found mainly in brackish 

 water. A few make their way far inland, but it is doubtful whe- 

 ther any species ' exists only in fresh water. The species seem to 

 be fairly numerous and individuals are often abundant. Their 

 habits are very uniform. They frequent submerged vegetation or 

 stones covered with algae and scrape therefrom the minute organ- 

 isms that form their food. Their mobile and extensile snouts 

 enable them also to feed easily on the algae that grow on the 

 shells of their companions and even on their own. The eggs are 

 sessile, relatively large and few. Some species are markedly gre- 

 garious. In eastern waters they apparently replace the Hydro- 

 biinae of the western parts of the Palaearctic Region. 



Subfamily STENOTHYRINAE. 



iSIS/. Stenoth\rinac, Kischer, Man. Cmicliy/ ml .. p. 7^4. 



Fischer includes in this subfamily of the Hydrobiidae two fos- 

 sil genera (Briartia and Nystia) as well as the living Stenothyra. 

 His description is short and he does not appear to have been 

 acquainted with some of the most characteristic features of the 

 shell or the animal. The subfamily, however, is well differentiated 

 from both the Bithyniinae and the Hydrobiinae and has no par- 

 ticular resemblance to the Mysorellinae. It may be redefined as 

 follows ; — 



Shrll minute, o\atL- or !.ubc\"lindric.il, compiessed in its dorso-ventral 

 axis: the aperture small, oblique ur transverse, oval or subcircular, witli 

 a complete and uniform peristome, which is never very prominent or in- 

 crassate. The surface smooth, rarely ornamented with periostracal 

 spines, more commonly with minute punctures arranged in spiral lines. 



OpERCr'Li'M horny but containing a considerable amount of calcareous mat- 

 ter, paucispiral externally, ivith tum pvominent trnnsverse ndges on tlie 

 internal surface. 



' S. foveolata. Benson, is on!\- known from SikrigaUi, a distance of 3(10 miles 

 from the sea and about 21.0 miles above the extreme tidal influence, but the spe- 

 cies may occur lower down as well in the (ianaetic Delta. 



