THE NATURAL HISTORY SURVEY. 127 
little prominences; dorsal and ventral margins rounded; an- 
terior margin long, broadly rounded; posterior margin short, 
rounded; umbonal slopes rounded; surface shining, lines of 
growth faint; color very light horn or yellowish; ligament short, 
dark horn; cardinal teeth small, a single, lamellar, elevated, 
arched tooth in the right valve; the right arm of the 
arch is longer than the left and reaches nearly to the ventral 
border of the hinge plate; the left valve is armed with two car- 
dinal teeth, the dorsal tooth being very long, lamellar, elevated 
and peculiarly bent at the distal end; the ventral tooth is large, 
elevated and more or less pyramidal; lateral teeth strong, 
curved, elevated, but projecting much above the edge of the 
valve, double in the right and single in the left valve; cavity of 
the beaks shallow. 
Length, 4.25; height, 3.00; breadth, 2.50 mill. (9883). 
Animal: Not observed. 
Distribution: New England west to California, Canada 
south to Louisiana, Florida and Honduras. > 
Geological distribution: Not known, probably Pleistocene. 
Habitat: Found in the smaller lakes and streams, buried 
in soft, black mud, or on aquatic plants. 
Remarks: This isa distinct and abundant species, distin- 
guished by its elongated ovate outline. It is quite abundant 
and is universally distributed. 
48. Pisidium virginicum Bourguignat, pl. xxviii., fig. 2. 
Tellina virginica GMELIN, Syst. Nat., pp. 32, 36, pl. clix., fig. 15, 1788. 
Tellina pusella (pars) Dittwyn, Cat., Vol. II., p. 106, 1817. 
Cyclas dubia Say, Nich. Encyc., lst Am. Ed. II., p. 4, pl. i., fig. 10, 1817. 
Physemoda aequalis RAFINESQUE, Ann. Gen. Sci. Phys. et Nat., Vol. V., p. 
319, 1820. 
Pisidium abruptum HaLpDEMAN, Proc. Phil. Acad., p. 53, 1841. 
Pisidium virginicum BOURGUIGNAT, Rev. Mag. Zodl., 1854. 
Shell: Of good size, thick, not much inflated, inequilateral, 
very oblique; umbones not much elevated, prominent, full, 
placed near the posterior end of the shell, marked by growth 
lines; dorsal margin almost straight; ventral margin rounded; 
anterior margin broadly triangular, long; posterior margin short 
and rounded, sometimes appearing as if truncated; umbonal 
slopes rounded, sometimes inclining to angular anteriorly; sur- 
face shining, marked by heavy growth lines which are faint on 
the protoconch; color dark horn in old specimens, lighter in 
younger specimens, and zoned with light yellowish, especially 
