THE NATURAL HISTORY SURVEY. 349 



the center cusp the largest; intermediate tooth squarish, much 

 produced at the outer, lower angle, and with the cusp finely 

 denticulated; lateral teeth long and narrow, finely denticulated 

 at the apex and partly down the sides (Fig. 131). 



Distribution: New England to Great Slave Lake, south to 

 Georgia and Louisiana. Dredged in Lake Superior at four to 

 thirteen fathoms. Alaska. (Randolph.) Manitoba. (Hanham.) 

 Geological distribution: Pleistocene; Loess. 



Habitat: Found plentifully in lakes, ponds and rivers, 

 where there is little or no current. 



Remarks: This is a very common species, easily distin- 

 guished by its discoidal form and rounded whorls. Like the 

 Limnaids it delights to float on the surface of the water, shell 

 downwards. It is very active and not at all timid in activity. 

 It seems to be confined to the southern and northern regions. 

 Hundreds of this species, as well as others, are thrown up on 

 the shore in North Chicago after a storm from the north. 

 144. Valvata tricarinata Say, pi. xxxii, fig. 14. 



Cyclostsma tricarinata SAY, Jour. Phil. Acad., Vol. I, p. 13, 1817. 



Valvata carinata SOWERBY, Gen. Sh., pi. xli, fig. 2. 



Valvata unicarinata DE KAY, N. Y. Moll., p. 118, pi. vi, fig. 129, 1844. 

 (Variety.) 



Valvata tricarinata var. simplex GOULD, Invert, Mass., p. 226, fig. 126, 

 1844. (Variety.) 



Tropidina carinata CHENU, Man. de Conch., Vol. II, p. 312, fig. 2232. 



Valvata tricarinata var. confusa WALKER, The Nautilus, Vol. XV, 

 p. 124, fig. 2, 1902. (Variety.) 



Shell: More or less turbinate, thin; color varying from 

 dirty white to horn-colored, translucent; surface shining, lines 

 of growth faintly marked, crowded; apex large, rounded, whit- 

 ish (or sometimes reddish), almost concealed in some speci- 

 mens by the volutions of the post-nuclear whorls; spire gener- 

 ally elevated; whorls three and one-half, strongly carinated, 

 rapidly increasing; the carinae are normally three in number, 

 one on the periphery, one on the shoulder of the whorls and 

 one on the base of the shell, but one or all of these may be 

 wanting; sutures very pronounced; aperture rounded, in some 

 specimens angled by the carinae, made continuous by a some- 

 what elevated columellar callus; columella straight, simple; 

 base rounded or keeled; umbilicus round and deep, funnel- 

 shaped when the base is carinated. 



Length, 4.00; width, 4.00; aperture length, 2.00; width, 2.00 mill. 



Animal: Similar to bicarinata. 



