[72] 



Inliablts the entire United States, but two species only on the 

 Pacific slope of the continent. 



**** Shell of moderate size, turbinate or depressed ; um- 

 bilicus closed ; aperture trigonal, lip with a long lam- 

 ellar tooth at base, and frequently a small tubercular 

 tooth above, a large curved lamellar p>arietal tooth. 



4, XoLOTREMA, Rafinesque. Shell uniform horn color, 

 depressed or. turbinate, frequently angulate or carinate 

 on the periphery ; base convex ; aperture with al\^jays a 

 lamellar curved parietal tooth, and a Ion/]; lamellar basal 

 tooth, with frequently also a small denticle on the 

 superior part of the lip. 

 Inhabits from the Alleghany to the Rocky Mountains. 



***** Shell moderate in size, aperture trilobate, caused by 

 denticles on the superior and inferior parts of the lip, 

 and on the parietal wall. 



, t Umbilicus open. 



• 5. Triodopsis, Rafinesque. Shell gl.-^bosely depressed, um- 

 bilicus open, lip teeth small, nearly equidistant. Uniform 

 horn color. 



ft Umbilicus closed. 



■G. IsoGNOMOSTOMA, Fitzinger. Shell smaller, globoscly 

 depressed, umbilicus covered by the extremity of the lip; 

 aperture three-lobed, the lip teeth small, the parietal 

 tooth larger, blade-shaped. Horn color, frequently 

 hirsute. 

 Eastern, Middle and Southern States, also Europe. 



****** Shell small, aperture -narrowly transverse, basal, 

 extending from the periphery to the axis of the shell ; 

 parietal wall with a long lamellar tooth, lip broad, with 

 generally a notch in the centre. 



'.T. Stenotuema, Rafinesque. Shell small, generally hirsute, 

 horn color, depressed turbinate above, very convex 

 below ; aperture narrow and lon^, basal, lip and parietal 

 wall subparallel, the former with a long blade-shaped 

 tooth, the latter either similar, Avith generally a notch in 

 the middle, or, the notch being wider, with two teeth ; 

 umbilicus closed. Within the aperture, and near the 

 axis, may be seen an accessory column or pillar, probably 

 ■designed to assist the animal in retiring within its shell. 

 United States, east of Rocky Mountains. 



