OF CONCHOLOGY. 155 



I 



Rissoidse. Shell elongated, generally costate or striate, heavy, 

 not umbilicated ; aperture angulated above, more or 

 less truncately and broadly channelled below, somewhat 

 rhomboidal, labrum very much callously thickened. 

 Generally white or brown, or banded. 



Operculum. Subspiral. 



Dentition. Rhachidian tooth with basal denticles at each 

 side. 



Foot projecting in front of rostrum. 



Distribution. Marine only. 

 Skeneim:. Shell depressed, planorboid. 



Operculum. Pauci-spiral. 



Dentition. Rhachidian tooth with the cusp large, obtuse; 

 laterals also different in form from Rissoidse. 



Distribution. Marine only. 



AMNicOLiDiE. Shell turbinate or elongate, generally smooth, 

 thin ; aperture generally round, and peristome continu- 

 ous, broadly rounded below, not thickened. Greenish. 



Operculum. Subspiral or concentric. 



Dentition. Rhachidian tooth with denticles placed on its 

 anterior surface behind the lateral margins. 



Distribution. Fresh water or amphibious. 



The reason why Dr. Stimpson did not find more than one 

 distinctive character to separate the families, it will be per- 

 ceived, is simply because he has included all the other charac- 

 ters in his diagnosis of Rissoidse. When we instituted the 

 family Amnicolidse, we had not studied the differences of the 

 Soft Parts, but simply those of the shells in connection with 

 their station ; and we still think that ample characters may be 

 thence derived, even if no differences in the animals had been 

 observed. 



The Skeneidse we now separate from Rissoidse, principally on 

 conchological grounds, as we are constantly receiving proof 

 of the vastly inferior value of the lingual armature as a dis- 

 tinctive character. With regard to Rissoidse, we adopt the 

 subfamilies Rissoininse and Rissoinse, but for the genera of 

 Amnicolidse we prefer a somewhat different grouping : — 

 Bythiniin^e. Operculum concentric, calcareous within. 



Stenothyrinje. Shell turbinate. Operculum subspiral, cal- 

 careous. Distribution Indian. Stenothyra, Gabbia* 



* This genus is correctly described in this Journal ("vol. i., p. 220, t. 

 22, f. 7) , but the figure of the operculum is not so distinctly drawn as could 

 be wished, which has caused Dr. Stimpson to doubt its generic position. 

 It is a good genus, and by no means an immature Stenothyra. 



