124 THE CHICAGO ACADEMY OF SCIENCES. 
ligaments slight; cardinal teeth small, a single, long, elevated, 
curved tooth placed near the dorsal border of the hinge plate in 
the right valve, and two teeth in the left valve, the dorsal tooth 
being long, elevated, lamellar and curved, and the ventral tooth 
elevated, pyramidal and stout; lateral teeth long and straight, 
elevated, lamellar, double in the right and single in the left 
valve; the anterior part of the valve is rounded, but the pos- 
terior forms a decided angle with the hinge line; muscle scars 
barely visible; cavity of the beaks deep; nacre horn color, with 
a blue zone on the ventral margin. 
Length, 6.00; height, 5.50; breadth, 3.00 mill. (8458). 
“550;  §.00; “ 3,00 “ (10098). 
if 8 50; See SCs (35 aS 4.50 ‘* (12466). 
Animal: Similar to that of S. occidentale. 
Distribution: New England west to Iowa, Wisconsin south 
to Alabama, Louisiana and Florida. 
Geological distribution: Pleistocene. 
ffabitat: Found in ponds and rivers, buried in black mud. 
Remarks: This species is closely allied to S. occidentale 
Prime, but may be distinguished by its larger size, truncated 
posterior border and general rounded oval outline. It is one 
of our commonest species, and also one of the most beautiful of 
the genus. /Partumeia is found in almost every part of the 
area, and is the most widely distributed of any species in the 
genus. It is a very handsome species when at its best, the shell 
being so thin that the animal may easily be seen through it. 
Genus PISIDIUM Pfeiffer. 
‘“Shell: Small, rounded-oval, inequilateral, anterior side 
longer; beaks terminal; cardinal teeth double, at times united, 
situated immediately under the beaks; lateral teeth elongated, 
lamelliform, double in the right valve, single in the left valve; 
ligament always on the shorter side. 
“Animal: Elongate-oval, compressed laterally; lobes of 
the mantle without tentacles, united posteriorly into a single, 
short siphonal tube; oral aperture small, tentacles of the 
mouth triangular, elongated; gills of medium size; foot small, 
tongue shaped, capable of great extension.’’ (Prime.*) 
“Posterior part of shell shorter than anterior; siphons quite 
short.” (Sterki, in lit.) 
*Monograph of American Corbiculade, p. 59. 
