40 EESEARCHES UPON THE HYDROBIIN^ 



bivalves. Something like this stylet I have observed also in our 

 American Melauians. 



In the following diagnoses of the genera, the characters given 

 are always those of the type, except when they are expressly 

 stated to have been made out from some other species. In the 

 description of the lingual dentition of the typical or a congeneric 

 species, the formula of the denticles is, strictly speaking, only 

 specific, and is given only as an indication of the generic numeri- 

 cal character of these denticles. Their respective limits as to 

 number in each genus must be determined by the examination of 

 the other species. 



STEIVOTHYRA, Benson. 



Nematura, Benson, in the Calcutta Journal of Science. Name pre-occu- 



pied in Ornithology. 

 Stenaihyra, Ben-son, in H. & A. Adams' Genera of Eecent Mollusca, II, 



(1858,) 626. 



Shell ovate, smooth, imperforate ; aperture rounded, contracted. 

 Operculum testaceous. 



Lingual ribbon, according to Troschel's figure,' with the rha- 

 chidian tooth only one-thii'd broader than long, and dee})ly emar- 

 ginated on either side beneath the cusp, which is armed with nine 

 denticles ; basal denticles two in number on each lateral margin, 

 pointing upward. Intermediate tooth with seven denticles ; inner 

 lateral with thirteen ; outer lateral with eleven. These denticles 

 may be conveniently formulated thus : 24^ - T - 13 - 11. 



Station, fresh-water. 



Distribution, India and Southeastern Asia, and the neighboring 

 islands. 



Type S. debtee, Benson. Syn. Nematura dcJtw, Benson, Calcutta Jour. 

 Sci.— H. & A. Ad. Gen. Rec. Moll. I, (18r)4,) 342, xxxvi, 5 (shell and oper- 

 culum). — Troschel, Geb. der Schnecken, I, (1857,) 104,vii, 11 (lingual den- 

 tition), Hab. River Ganges. 



The species are enumerated in H. & A. Adams' " Genera," II, 

 626, and by von Frauonfcld in the " Verhandl. der k.-k. zool.-bot. 

 Gesellschaft in Wien." XII, (1862,) p. 1158. 



This genus is phiced in the Yiviparidaj by H. & A. Adams, 

 von Frauenfeld, and most other authors, and in the Littorinidae 



' Gebiss der Schnecken, I, pi. vii, fig. 11. 



