AND ALLIED lOIl.VS. 



21 



isogona?' and "A. Integra'^ have in fact been placed in this 

 genus bj Frauenfold.* 



Fig. 10. 



Genus SOMATOCiVRUS, Gill,* 



T am indebted to the kindness of Mr. Tryon, and of Professor 

 Sheldon, of Davenport, Iowa, for alcoholic specimens of S. de- 

 pressus (Fig. 10), the species upon which this genus 

 was founded. Very singularly, however, among 

 these specimens, more than a hundred in number, I 

 can find no males, and am therefore unable to describe 

 the form of the verge in the type. 



The genus presents the following characters : Shell 

 short, thin, simply striate, distinctly umbilicated, and 

 with four to six whorls ; the body-whorl subglobose, more or less 

 shouldered above; the spire small, and the suture impressed. 

 Aperture oblique (upper part most advanced), rhombo-ovate, 

 narrowly rounded in front and behind ; peritrerae thin and acute, 

 appressed behind, below the upper angle, to the whorl, and with 

 its entire margin in the same plane. Operculum subspiral, corne- 

 ous, but comparatively thick and strong, and with its inner margin 

 regularly convex. Foot short. Snout robust, and considerably 

 longer than in Amnicola. Tentacles tapering, pointed. 



The lingual dentition of the type is as follows (Fig. 11) : Rha- 



chidian tooth short and very broad, and trilobate below, with 

 the outer angles much produced and narrow; cusp armed with 

 seven denticles ; basal denticles four on each side, the innermost 

 largest, but not reaching the inferior margin of the tooth, and 



' Verhandl. der K.-k. Zool.-bot. Gesellschaft in Wien, XIII, 1863, 194, 

 195. 

 " Proc. Acad. Nat. Sci. Phila., 1863, p. 34. 



