2 ANCYLUS. 



capable of a little extension beyond the shell, 

 and of turning nearly at right angles to it. 



SHELL patelliform, simple, sinistral, with the 

 apex directed backwards, and towards the right: 

 pallial impression submarginal, interrupted upon 

 the left side. 



EXAMPLE. Ancylus rivularis, Say. 



HABITS. Attached to the sides and lower 

 surface of stones; or upon and within dead 

 bivalve shells; either in running or quiet water. 

 The American species, as far as can be ascer- 

 tained, appear not to require access to the 

 atmosphere. PHYSAD^E, p. 11. 



OBSERVATIONS. 



In the genus Siphonaria the pallial impression 

 is interrupted in front, and Patella has the apex 

 directed forwards. 



The breathing aperture is upon the left side, 

 next the labrum, or outer lip, as in Physa; 

 which it also resembles in having the apex 

 directed to the right. The tongue is long, 

 slender, and covered with minute teeth, of 

 which there are about eighty rows, and thirty 

 teeth in each row, in A. rivularis; the only 

 species I have examined. The edges of the 

 mouth and head are covered with vibrillse. 



