52 RESEARCHES UPON THE HYDROBIIN.E 



and with its entire margin uniformly in one plane, the outer 

 lip not projecting anteriorly. Operculum rather thick, corneous, 

 subovate ; inner margin concave near the upper extremity. 

 Foot rather short. Rostrum bi'oad. Tentacles tapering, pointed. 



Lingual dentition of type : Rhachidian tooth very short and 

 broad. Intermediate tooth with the body perforated. Inner and 

 outer lateral teeth with about the same number of denticles. 

 Formula of the denticles : j-j , -*r-14-14. 



Station, fresh water. 



Distribution, the central parts of North Americn. 



Type S. depressus, Gill, loc. cit., 1863, p. 34. — Amnicola depressd, Tryon, 

 Proc. Acad. Nat. Sci. Phila., 1862, p. 452, wood-cut. Iowa. 



As suggested on a previous page, the Ilelania isogona of Say, 

 in the "New Harmony Disseminator," Vol. II, (1829) p. 277, 

 will prove to belong to this genus, if the typical species, S. de- 

 pressus, is found to have a broad, compressed, and bifid verge. 



AM\^ICOI.A, Gould & Hald. 

 Amnicola, Gould & Hald., in Gould's Invert, of Mass., (1841) p. 228. 



Shell small, rather short, ovate or subglobular, thin, smooth, 

 perforate ; spire not acute. Aperture broadly ovate, not oblique ; 

 outer lip thin and sharp, not projecting anteriorly. Operculum 

 corneous. Foot rather short and broad, expanded and broadly 

 rounded behind. Rostrum short. Tentacles cylindrical, blunt 

 at their tips. Verge short, bifid, with a globular base. 



Lingual dentition of A. porata: Rhachidian tooth very short 

 and broad, with a tongue-shaped process from the middle of the 

 anterior surface, reaching beyond the base. Intermediate tooth 

 with a short broad body having a strongly projecting infero- 

 interior angle, and a very long peduncle. Formula of the den- 

 ticles : ^;^- 5- 18-30. 



Ova-capsules semi-lenticular in form, with a laminiform limb. 

 Each contains but one a^g. 



Station, fi-esh water. 



Distribution, North America. 



Type A. limosa, Haldemax, Monog. Amn., p. 10, pi. i, fig. 5, 6. — Paht- 

 dlna limosa. Say, Jonrn. Acad. Nat. Sci. Philad., I, 125. — Amnicola porala, 

 Gould, Inv. Mass., (1841) 229, fig. 157. New England States. 



