MESODON. 323 



presence does not seem to cause any uneasiness, nor even to be felt by the 

 snail. 1 



Mesodou Mitchelliana, LEA. 

 Vol. III. PL IV. outline figs. 



Shell imperforate, depressed conoid-globose, thin, with crowded striae and 

 very crowded decussating microscopic lines, pellucid, horn-color, polished ; spire 

 briefly conoid ; whorls 5, moderately convex, gradually increasing, the last ven- 

 tricose, subconstricted and briefly deflected anteriorly; aperture diagonal, lunate, 

 sub-pearleaceous within ; peristome white, thickened, its terminations slightly 

 converging, subequally reflected, that of the columella narrow, adherent, or 

 subdilated and spreading. Greater diameter 16^, lesser 14^ mill.; height, 

 10 mill. 



Helix Mitchelliana, LEA, Am. Phil. Trans., VI. 87, PL XXIII. Fig. 71 ; Obs., 

 II. 87 (1839); TROSCHEL, Arch. f. Nat., 1839, II. 221. DEKAY, N. Y. 

 Moll., 45 (1843). PFEIFFER, Mon. Hel. Viv., I. 291 ; IV. 322. BLAND, 

 Ann. N. Y. Lye., VI. 339 (1858). W. G. BINNEY, Terr. Moll., IV. 47 ; L. & 

 Fr.-W. Sh., I. 141 (1869). 



Helix clausa, BINNEY, Terr. Moll., II. 109 ; in Vol. III. PL IV, outline figures. 



Mesocbm, Mitchelliana, TRYON, Am. Journ. Conch., III. 45 (1867). 



Kentucky and Ohio, along the Ohio River; Munroe County, Virginia; 

 Cherokee County, North Carolina. A species of the Interior Region. 



In M. clausa the umbilical region is more widely excavated, and the groove 

 behind the reflected peristome producing the contraction of the aperture, is 

 continued at the base of the shell, becoming wider as it joins the umbilical 

 opening. In M. Mitchelliana the groove is almost obliterated at the point of 

 reflection of the peristome over the umbilicus, by the more tumid character of 

 the last whorl. 



Jaw arcuate, of uniform width throughout ; ends blunt ; anterior surface with 

 12 crowded, coarse ribs, denticulating either margin. 



Lingual membrane (PL VIII. Fig. H) with 491 49 teeth; 18 laterals. 

 Outer laterals have side cusps and cutting points. 



The genital system is long and narrow. The oviduct is greatly convoluted. 

 The penis sac is long, stout, cylindrical, with a bulb-like expansion at its apex, 

 at which point both vas deferens and retractor muscle are inserted. The 

 genital bladder is lengthened, ovate, not much larger than its duct, which is 

 short, and enters the vagina below the middle of its length (PL XI. Fig. H). 



l Hypopus concolor, HALDEMAN. Oval, nearly colorless or very pale 

 ochraceous ; bristled ; sides impressed. Length, 0.4 millim. 



Differs in outline from the European species, which it resembles in 

 general appearance, mode of life, .and in the large pair of projecting 

 setse anteriorly and posteriorly. A colored dorsal line has been ob- 

 served. 



