[81] 



believing that major is only allolahris living in situations 

 highly favorable to its growth, and gives measurements of 

 specimens from different northern and southern localities, 

 showing a gradual increase in size, as well as variations of 

 form. I believe that the tme rrajor inhabits the far Southern 

 States, where it replaces albolabris, and I doubt very much 

 whether it was ever found North of Southern Tennessee. 



10. Mesodon albolabris, Say. 



Plate 7, figures 5, 6, 7. 



Depressed orbicular, moderately thick, closely obliquely 

 striate, with crowded, slightly- impressed revolving lines; 

 spire convex, suture not deeply impressed ; whorls 5^ — 6, flat- 

 tened convex, the body a little deflected at the aperture, and con- 

 tracted behind the lip ; mouth lunar, with a widely-reflected 

 white lip, which, at the base, covers the umbilicus. Yellowish 

 brown or light chestnut color. 



Variety with a parietal denticle. 



Diam. 25, height 15 mill. 



Inhabits from Canada to South Carolina, and westward to 

 Arkansas and Nebraska. 



One of our mogt common species. This, as well as all others 

 of the group, when immature, is furnished with an open um- 

 bilicus and a sharp unreflected lip ; and in this state it is ex- 

 ceedingly difficult to distinguish the species one from another. 



11. Mesodon Pennsylvanica, Greea. 



Plate 8, figure 9. 



Turbinately subglobose, moderately thick, translucent, with 

 crowded, elevated oblique striae; spire convexly elevated, 

 suture distinct ; whorls 6, convex ; aperture subtriangular, 

 contracted behind the lip, which is white, and narrowly 

 reflected, and slightly thickened internally at the base; um- 

 bilical region indented, umbilicus covered. Bright horn color. 



Diam-. 13—18 mill. 



Western Pennsylvania, Ohio, Illinois. 



This species is distinguished from M. claum by its imper- 

 forate base and triangular aperture, and from thg following 

 species by its more turbinate form, as well as by the aperture. 



