•0, 



[72] 



Inhabits 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 parietal tooth. 



4. XoLOTREMA, Rafinesque. Shell uniform horn color, 

 depressed or turbinate, frequently angulate or carinate 

 on the periphery ; base convex ; aperture with always a 

 lamellar curved parietal tooth, and a long lamellar basal 

 Iwoth, 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. 



Triodoi'Sis, Rafinesque. Shell gl.^bosely depressed, um- 

 bilicus open, lip teeth small, nearly equidistant. Uniform 

 horn color. 



ft Umbilicus closed. 



6. IsoGNOMOSTOMA, Fitzinger. Shell smaller, globosely 

 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. 



1. Stenotrema, Rafinesque. Shell small, generally hirsute, 

 horn color, depressed turbinate above, very convex 

 below ; aperture narrow and long, basal, lip and parietal 

 wall subparallel, the former with a long blade-shaped 

 tooth, the latter either similar, with 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. 



