128 THE CHICAGO ACADEMY OF SCIENCES. 
near the ventral border; ligament as usual; cardinal teeth small, 
a single, elevated, arched tooth in the right valve, of which the 
right arm of the arch is the longest and reaches to the ventral 
border of the hinge plate; both arms are swollen at the ends 
and the arch forms as a whole, a letter V reversed; the left 
valve is armed with two teeth, the dorsal being thick, long, 
elevated and straight, and reaches nearly to the ventral edge of 
the hinge plate; the ventral tooth is large, rounded and pyram- 
idal; lateral teeth strong, curved, elevated, smooth or only 
slightly striated, those of the left valve projecting strongly 
above the valve edge, those of the right valve not so much so, 
but projecting into the cavity of the shell; double in the right 
and single in the left valve; cavity of the beaks shallow; nacre 
glossy, inclining to purplish. 
Length, 6.00; height, 5.00; breadth, 4.00 mill. (9879). 
Animal; Not examined. 
Distribution: New England west to Iowa, Canada south to 
Louisiana. - 
Geological distribution: Pleistocene. 
Habitat: Same as abditum. 
Remarks: A species easily recognized ‘by its large size (it 
is the largest of the genus) and oblique form. It is one of the 
handsomest of the Pisidia. Thus far it has only been found in 
the southern region. Found fossil in sand banks east of Sheri- 
dan Drive and north of Graceland Avenue. 
49. Pisidium compressum Prime, pl. xxviii., fig. 7. 
Cyclas altilis ANTHONY, in litt., 1847. 
Pisidium compressum PRIME, Proc. Bost. Soc. N. H., Vol. 1V., p. 164, 
1851. 
Pisidinm cicer PriME, Ann. N. Y. Lyc., Vol. VI., p. 65, pl. i., fig. 1, 1853. 
Shell: Small, solid, inflated, subequilateral, very oblique, 
trigonal; umbones elevated, drawn up and compressed, placed 
near the posterior end of the shell, marked by lines of growth; 
there is a wing shaped appendage on the summit of the beaks; 
dorsal margin very arcuate; ventral margin rounded; anterior 
margin long, narrow, produced; posterior margin short and 
rounded; umbonal slopes rounded; surface shining, marked by 
coarse growth lines; color yellowish, darker in old specimens; 
ligament as usual; cardinal teeth small, a single, long, curved 
tooth extending from near the ventral to the dorsal edge of the 
hinge plate, the right arm of the arch being long and swollen; 
. 
