330 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



bulb, into which the vas deferens enters. The retractor muscle is inserted 

 above the entrance of the vas deferens. 



Mesodon Wetherbyi, BLAND. 



Shell with umbilicus covered, orbicular-depressed, thin, granulately striate, 

 pale horn-colored ; epidermis dark, covered with oblique, prostrate hairs ; spire 

 somewhat conoidal, suture impressed, apex obtuse ; whorls 5, 

 Fig. 213. slightly convex, gradually increasing, the last suddenly de- 



flected, rather gibbous, constricted, beneath convex, subangu- 

 late at the periphery ; aperture oblique, roundly lunate, with 

 a white, erect, oblique, tongue-shaped parietal tooth ; peri- 

 stome thickened, angularly reflected, the upper margin ex- 

 panded, the columellar margin dilated, covering the umbilical 

 perforation. Greater diameter 17, lesser 15 mill.; altitude, 

 8 mill. 



M. Wetherbyi. Helix Wetherbyi, BLAND, Ann. Lye. Nat. Hist. N. Y., X. 361, 



1873. 



At the base of sandstone cliffs, mouth of Laurel River, Whitley County, 

 Kentucky. Probably a species of the Cumberland Subregion. 



This species belongs to the same group as dentifera, Binney, and Roemeri, 

 PfeifFer, but is of smaller size, somewhat more elevated, and readily distin- 

 guished from them by the sculpture and epidermis. It differs from M. divesta, 

 Gould, in having a parietal tooth, and, although in general appearance like a 

 small form of M. appressa, Say, is without the lamina on the basal margin of 

 the peristome. (Bland.) 



Jaw as usual in the genus; about 18 ribs. 



Lingual membrane (PI. VIII. Fig. D) with 35 135 teeth ; 12 laterals. 

 It will be seen in the figure that the marginal teeth have a simple, not bifid, 

 inner cutting point, a peculiarity shared by only a few other species. 



Genitalia unobserved. 



Mesodon thyroides, SAY. 

 Vol. III. PI. XL 



Shell narrowly umbilicated, depressed globose ; spire convex ; epidermis of 

 a uniform yellowish-brown or russet color ; whorls 5, with fine, parallel striae, 

 running obliquely across them ; spire more or less elevated ; suture distinctly 

 impressed ; aperture lunate, contracted by the peristome, the plane of the aper- 

 ture making a considerable angle with the plane of the base of the shell ; pari- 

 etal wall with a prominent, white, tooth-like process placed obliquely to the axis 

 of the shell ; peristome white, thickened, widely reflected, and sometimes grooved 

 on its face, its exterior yellowish ; umbilicus exhibiting only one volution, par- 



