OF CONCHOLOGY. 47 



covering the "umbilicus ; base very convex. Yellowish horn 

 color. 



Diam. 16, height 10 mill. 



Oregon. 



Peculiar in this group, for its very close arrangement of 

 short bristles all over the surface ; the old shells are frequently 

 denuded of these, but then the scars of their insertion are dis- 

 tinctly visible with a glass. As already mentioned, this spe- 

 cies sometimes developes a strong, oblique, parietal tooth, 

 (fig. 12). 



§§ Not hirsute. 

 17- Mesodon Downieana, Bland. 



Plate 8, figure 15. 



Subglobose, thin, subpellucid, with obsolete rib-like striee 

 and crowded minute revolving lines ; spire depressed conical, 

 suture moderately impressed; whorls 5, convex, the last 

 tumid, scarcely descending, constricted behind the lip ; aper- 

 ture obliquely lunate, lip white, reflected narrowly, nearly 

 covering the umbilicus ; base convex. Greenish horn color. 



Diam. 10*5, height 6 mill. 



University Place, Franklin County, Tennessee. 

 Like M. Ohristyi, Bland, in form, but has no parietal tooth. 



18. Mesodon clausa, Say. 



Plate 8, figure 16. 



Subglobose, moderately thick, with fine oblique stria? ; spire 

 convex, with distinct suture ; whorls 5, convex, the last large, 

 contracted behind the lip ; aperture rounded, lip white, nar- 

 rowly reflected and nearly covering the umbilicus ; base very 

 convex. Light yellowish brown, shining. 



Diam. 13 — 15 mill. 



Inhabits all the Western States from the Great Lakes to 

 Mississippi and Alabama. 



This species is of the same size as M. Pennsylvaniea and 

 M. Mitchelliana, inhabiting partially the same region. Its dis- 

 tinctive characters have been already pointed out. It is most 

 abundant in the southwest in a semi-fossil condition. 



