MESODON. 333 



Hel. Viv., I. 291 ; in Chemnitz, ed. 2, II. 51, ex parte. — Reeve, ex parte, 



Con. Icon., No. 676 ; not of Green. 

 Helix Mitchelliana, Pfeiffer in Chemnitz, 1. c. I. 332, PI. LVI. Figs. 6-8. 

 Mesodon clausa, Tryon, Am. Journ. Conch., III. 47 (1867). 



A Post-pleiocene species now found in the Interior Region, in the States bor- 

 dering on the Ohio River, and in Wisconsin, Missouri, Tennessee, Mississippi, 

 and Alabama. 



Animal blackish. 



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. 



H. Ingallsiana, Shuttleworth (Jugallsiana of Albers ed. 2), is a small form 

 of clausa. I give a figure copied trom an unpublished plate of 

 Shuttleworth. F *- 216 - 



Jaw as usual in the genus ; about 10 stout ribs. 



Lingual membrane as in M. thyroides (PI. VII. Fig. T) ; it has 

 41 — 1 — 41 teeth, with about 11 perfect laterals. I can detect 

 no side cusps, even on the extreme outer marginals. (Shuttleworth) 



The genitalia are figured on PI. XIV. Fig. G. The penis sac 

 is the conspicuous feature of the system : it is longer than the oviduct, and 

 almost as stout, of about equal size throughout ; it has the entrance of the 

 vas deferens and retractor muscle at its blunt apex. The genital bladder is 

 small, lengthened oval, with along, slender duct. The prostate is narrow, stout, 

 prominent, cordlike. The vas deferens is large. The other organs present no 

 peculiar features. 



Mesodon Columbiana, Lea. 

 Vol. III. PI. V. 



Shell umbilieated, subdepressed-globose ; epidermis with short, rigid hairs, 

 corneous, thin ; whorls 6, slightly rounded, very minutely striated, rising grad- 

 ually, but regularly, one above the other to an acuminated apex ; sufure 

 strongly impressed ; aperture roundly lunate, a little contracted and thickened 

 by a testaceous deposit or border, at the angle of reflection of the peristome ; 

 peristome thickened, whitish, or brownish white, reflected but not flattened, 

 rather grooved on its face, the basal margin horizontal in its direction, with a 

 slight thickening or projection before it reaches the base of the shell ; umbili- 

 cus open, partially hidden by the reflected peristome at its junction with the 

 base; base a little flattened. Greater diameter 17, lesser 14 mill.; height, 11 

 mill. 



Helix Columbiana, Lea, Am. Phil. Soc. Trans., VI. 89, PI. XXIII. Fig. 75 ; Obs., 

 II. 89 (1839); in Troschel, Arch. f. Nat., 1839, II. 221. — DeKay, N. Y. 



