ANCYLUS. 6 



The allied genus Velletia, separated by Mr. 

 Gray, is dextral; but as the pallial impression 

 is obsolete in delicate shells, and the apex fre- 

 quently so much eroded that its proper direction 

 cannot be ascertained, it is sometimes extremely 

 difficult to decide between the two genera. The 

 species figured upon plate 1, appear to be true 

 Ancyli, no native species of Velletia having yet 

 come under my observation. 



Specific distinctions are but slightly marked 

 in the Physadce; and in a genus like Ancylus, 

 are particularly difficult to discover, from the 

 great similarity of the spire and aperture; and 

 the absence of the suture and columella, with 

 the consequent variety of character which they 

 present. Hence, the danger of multiplying spe- 

 cies is greatly increased, especially in cases 

 like the present, where several are characterized 

 from a small number of individuals; and they 

 must, consequently, be admitted with caution. 



The outline figures (marked a) upon the plate, 

 represent the natural size of the shell in profile; 

 and the dimensions are given as affording addi- 

 tional means of identification. This has not 

 been deemed necessary with those genera which 

 are figured the natural size. 



