78 THE CHICAGO ACADEMY OF SCIENCES. 
equal in the left valve, and the anterior tooth small and rudi- 
mentary in the right, all stout, heavy, a trifle elevated, 
triangular, very heavily sulcated; lateral teeth long, wide, thick, 
heavy, pyramidal when studied in cross-section, serrated, 
directed toward the ventral margin; connecting plate between 
cardinal and lateral teeth flat, thick, not much spreading; 
anterior adductor muscle scar mussel shaped, very deeply exca- 
vated, concentrically and longitudinally striated; posterior 
adductor muscle scar rounded, impressed, concentrically striated; 
protractor pedis muscle scar wider than long, deeply impressed, 
striated; dorsal muscle scars situated on the posterior face of the 
cardinal teeth, deeply impressed; pallial line crenulated, 
impressed; cavity of the beaks deep; nacre silvery white with a 
tinge of salmon near the cavity of the beaks, and on the anterior 
part, iridescent. 
Length, 64.00; height, 45.00; breadth, 27.00 mill. (8869). 
69.00;  *" 49.00; = «80-50 (7764). 
“40.00; ‘ 80.00; ‘ 22.50 ** (7764). 
“ 7900; ‘ 60.00; ‘ 33.00 ‘* (18441). 
Animal; Generally brownish slate color, except on the 
foot which is dark flesh or salmon; ctenidia short and wide, 
rounded before and behind, united above, and to the opposite 
pair, throughout their entire length, the inner one the larger; 
color of ctenidia bluish; labial palpi tongue shaped, united at 
base and for a short distance posteriorly; foot very thick; 
siphonal openings brownish, the anal small and almost smooth; 
the branchial larger, with dark flesh or salmon colored papille; 
edge of mantle brown; liver yellowish brown; between the 
beaks and the pericardium the animal is bluish white. The 
abdomen is of a beautiful pearl color, and the mantle is some- 
times pink or salmon colored. Four gills used as marsupia. 
Distribution: New York west to Iowa, Michigan south to 
Louisiana and Alabama. 
Geological distribution: Pleistocene. 
Habitat: Found rather plentifully in the larger rivers, on a 
muddy bottom, in water from a foot to twenty feet or more in 
depth. 
Remarks: A species showing considerable variation, espe- 
cially in the shape of the posterior portion, and in its corpulency. 
It is always dark colored. Some specimens approach very 
closely to ¢v/gona. The form is also confounded, by many con- 
chologists, with coccinea, but that species is more rounded and 
