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THE CHICAGO ACADEMY OF SCIENCES. 
Shell: Very fragile, small, inflated, almost equilateral, rhom- 
bic-ovate, translucent; umbones prominent, elevated, full, 
calyculate, approximating, placed centrally, smooth and shining; 
dorsal margin straight; ventral margin broadly rounded; anterior 
margin rounded; posterior margin sharply truncated, rounded 
on the ventral part; umbonal slopes rounded; surface smooth 
and shining, lines of growth very fine; color light yellowish 
green or greenish horn with a zone of yellow bordering the ven- 
tral margin of the valve; ligament weak, light horn color; car- 
dinal teeth small, a single, elevated, lamellar, arched tooth in 
the right valve, and two teeth in the left valve, the ventral tooth 
pyramidal, elevated, the dorsal tooth long, lamellar, curved, 
and elevated; lateral teeth long, lamellar, elevated, straight, one 
in the left valve and two in the right valve; muscle scars 
scarcely visible; cavity of the beaks shallow; nacre light bluish 
with a yellow zone on the ventral border. 
Length, 9 25; height, 7.50; breadth, 5.00 mill. (12464). 
Paes ta 6.50; 7 B00 i (12464). 
Animal: Not observed. 
Distribution: New England west to Illinois and Wisconsin, 
Canada south to Kentucky. 
Geological distribution: Pleistocene. 
Habitat: In small ponds and sheltered parts of rivers, in 
soft, sticky mud. 
Remarks: This species is very like S. ¢ransversa, but is 
shorter in comparison with its height, is rhombic in form and 
the beaks are placed centrally. The two species belong toa 
natural group of which ¢ransversa is the leading form. It 
is not as common as /ransversa and is confined to the lower 
part of the western region. 
45. Calyculina securis Prime, pl. xxvii., fig. 9. 
Cyclas securis Prime, Proc. Bost. Soc. Nat. His., Vol. IV., p. 160, 1851; 
Ann. N. Y. Lyceum, Vol. V., p. 218, pl. vi., 1851. 
Cyclas cardissa PRiME, Proc. Bost. Soc. N. H., Vol. IV., p. 160, 1851. 
Cyclas crocea Lewis, |. c., Vol. V., p. 25, 1854. 
Shell: Small, fragile, but stouter than the two previous 
species, inflated, inequilateral, rhombic-orbicular; umbones 
elevated, full, much inflated, calyculate, approximate, placed a 
trifle anteriorly; marked by very fine lfmes of growth; dorsal 
margin arched; ventral margin rounded; anterior margin 
rounded, posterior truncated; umbonal slopes rounded, sub- 
