1z0 THE CHICAGO ACADEMY OF SCIENCES. 
face shining, marked by very fine lines of growth; color light 
horn, sometimes darker; ligament as usual; cardinal teeth 
small, a single, elevated, lamellar, curved tooth in the right 
valve, the posterior curve of which is longer than the anterior 
and is club shaped, and two teeth in the left valve, that near the 
ventral border of the hinge plate being elevated and pyramidal, 
that on the dorsal border being long, lamellar, depressed, and 
curved, as in the preceding species; lateral teeth short, elevated, 
curved, single in the left and double in the right valve; muscle 
scars scarcely discernible; cavity of the beaks shallow; nacre 
light purplish or bluish. 
Length, 7.50; height, 7.00; breadth, 4.50 mill. (8456). 
4600302 re BOOS! ow 8GDa “OAganED: 
Animal: Transparent, whitish, reddish brown near the um- 
bones; mantle white, thin and transparent; siphons of medium 
length, united at base; ctenidia large, wide, rounded, united 
above; labial palpi long and narrow, united at base; foot large, 
long and narrow, extended from the center of the shell; the shell 
is so transparent that most of the organs may be seen through 
it when held between the observer and the light. 
Distribution: New England west to Washington, Canada 
south to Louisiana. 
Geological distribution: Pleistocene. 
Habitat: Found in lakes and rivers, in soft mud, either on 
the surface or buried. 
Remarks: This species is distinguished by its oval outline, 
which is more regular than that of any other spherium found 
here. It is very common, and when found at all is usually rep- 
resented by hundreds of individuals. 
Genus CALYCULINA Clessin. 
“Posterior part longer, or scarcely so, higher than anterior; 
shell very thin and fragile; hinge very fine.” (Sterki in lit.) 
43. Calyculina transversa Say, pl. xxvii., fig. 5. 
Cyclas transversa Say, New. Harm. Dissem., Vol. II., p. 356, 1829. 
Cyclas detruncata PRIME, Proc. Bost. Soc. N. H., Vol. IV., p. 155, 1851. 
Cyclas gracile Prime, |.c., Vol. IV., p. 156, 1851. 
Cyclas constricta ANTHONY, l.c., Vol. IV., p. 274, 1852. 
Shell: Thin, of good size, inflated, inequilateral, trans- 
versely oblong, translucent; umbones elevated, not large, full, 
calyculate, placed anterior to the center of the shell, smooth and 
