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. Ss ek 
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 
Limneids 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, pl. xxxii, fig. 14. 
Cyclostsma tricarinata SAY, Jour. Phil. Acad., Vol. I, p. 18, 1817. 
Valvata carinata SOWERBY, Gen. Sh., pl. xli, fig. 2. 
Valvata unicarinata DE Kay, N. Y. Moll., p. 118, pl. 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. 2282. 
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 carinze 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 carine, 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 dicarinata. 
