330 HELICID.E. 



REMARKS. This is a very variable species. The 

 ordinary specimens vary chiefly in the armature of 

 the aperture, the marginal, internal rim of calcareous 

 matter thickening with age, and developing more numer- 

 ous denticles. The Ohio specimens are of more than 

 ordinary size, clean and shining, and were the form 

 designated by Prof. Adams as P. Tappaniana. Those 

 found in Massachusetts are considerably smaller, covered 

 with a well developed epidermis, and often, if not always, 

 have the aperture decidedly modified in form, being more 

 triangular, and the denticles more or less curved. To 

 these was applied the name curvidens ; and the modifi- 

 cations are so constant as to incline us still to regard 

 them as constituting a distinct species. With all its 

 variations, it has an aspect which enables us readily to 

 separate it from all other species. The form of the shell 

 itself, and its semicircular aperture, are sufficiently pecu- 

 liar. A more careful examination of the animal shows 

 decidedly that it does not belong to Vertigo, as supposed 

 by Mr. Say. [G.] 



