34 AMERICAN JOURNAL 



MONOGRAPH OF THE TERRESTRIAL MOLLUSCA OF THE 

 UNITED STATES. 



BY GEORGE W. TRYON. 



[Continued from p. 327, vol. ii.] 



MESODONTIN^. 



Genera. 



* Shell minute, margin of lip circular, its extremities 

 approaching and co7ineeted hy a callus : not toothed. 



1. Vallonia, Risso. Shell minute, diaphanous, umbilicate ; 



lip margin broadly reflected, nearly circular, white. 



European ; extending into the States east of the Rocky 

 Mountains. 



** Shell large, umbilicate, lip with a small tooth at its 

 base ; sometimes also with a small parietal tooth. 



2. Ulostoma, Albers. Shell large, globosely depressed, 



aperture semicircular, lip tuberculately toothed at base- 

 Horn color, sometimes banded with rufous. 



Alleghany Mountains, from Vermont to Tennessee, and west- 

 ward to the Rocky Mountains. 



*** Shell large, umbilicus covered or perforate, lip not 

 toothed ; with geyierally a small oblique imrietal tooth. 



3. Mesodon, Rafinesque. Shell large, subglobose or orbicu- 



larly depressed ; aperture rounded lunar, the lip some- 

 times slightly dentately thickened at the base ; parietal 

 tooth, when present, small ; umbilicus either covered by 

 an expansion of the lip or partially covered. Generally 

 uniform pale horn color. 



