HELIX. 113 



Subsection I. TRIODOPSIS (sensu stricto). 



Umbilicus open, lip teeth small, subequal. H. TRIDENTATA, Say. 



United States. 



Subsection II. ISOGNOMONOSTOMA, Fitzinger, 1833. 



Umbilicus closed, parietal tooth often larger, blade shaped, 

 epidermis sometimes hirsute. H. INFLECTA, Say. 



United States, Europe. 

 Helicodonta, Moquin Tandon, 1855, is a synonym. 



Subsection III. XOLOTREMA, Rafinesque, 1819. 



Umbilicus closed, aperture with a lamellar curved parietal tooth, 

 a long lamellar tooth on the basal part of the lip, and frequently a 

 small denticle on its upper part. H. PALLIATA, Say. 



United States. 



Subsection IV. METODONTIA, Mollendorff, 1883. 



Narrowly umbilicated, or nearly covered, lip dentately thickened 

 within, parietal wall also dentate. 

 H. HEMIPLEURIS, Moll. China. 



Section VI. MESODON, Rafinesque, 1819. 



Shell rather large, depressed globose, usually light horn color, 

 the aperture with a white reflected lip, lip with or without a single 

 basal tooth, parietal wall with or without a small denticle, umbilicus 

 open or closed. 



Subsection I. MESODON (restricted). 



Shell subglobose or orbicularly depressed ; the lip sometimes 

 slightly dentately thickened at the base ; parietal tooth, when 

 present, small ; umbilicus partially or completely closed by an 

 expansion of the lip. 



H. ALBOLABRIS, Say. United States. 



Aplodon, Raf., 1819, and Patera, Albers, 1850, are synonyms. 



Subsection II. ULOSTOMA, Albers, 1850. 



Shell large, globosely depressed, aperture semicircular, the lip 

 tuberculately toothed at the base, umbilicus open. 



H. SAYI, Binney, United States. 



Section VII. ATOPA, Albers, 1850. 



Often sinistral, planorbiform, flattened above, whorls more con- 

 vex below, widely umbilicated, the last whorl more or less compressed 



and deflected in front, aperture oblique or subhorizontal, the lip 

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