THE NATURAL HISTORY SURVEY. 63 
tor muscle scar long, rather wide, deeply impressed, marked by 
lines of growth; posterior adductor muscle scar wide and 
spreading, not much impressed; protractor pedis muscle scar 
not heavily impressed, marked by lines of growth, wider than 
long, and connecting with the anterior adductor muscle scar 
without a break; dorsal muscle scars situated on the anteriorface 
of the cardinal teeth and in the cavity of the beaks, large and 
deeply pitted; pallial line impressed; cavity of the beaks shal- 
low; nacre bluish white, pearly, and iridescent on the edges of 
the valves. 
Length, 75.00; height, 42.00; width, 34.00 mill. (7159). 
“61.00; ' 87.00; ‘ 25.00 “ — (7160). 
OSs OON mines 28.00% ite, J 20FO0. <S (7161). 
Animal: Anal opening rather large, without papille; 
branchial opening very small, with delicate brown papille; 
palpi not united at posterior edges; branchie large, curved 
below, inner much larger, united to abdominal sac all the way 
or partly free. Outer gill of female occupied throughout with 
embryos. (Simpson.) 
Distribution: ‘Western New York west to Iowa, Michigan 
south to Louisiana. 
Geological distribution: Pleistocene. 
Habitat: In rivers and lakes, ona muddy bottom, in from 
two to fifteen or twenty feet of water. 
Remarks: A species at once distinguished by its heavy, 
inflated shell and peculiarly marked unbones, the latter being 
larger in proportion to the size of the shell than in any species 
found in the region. In margina¢a the lower posterior angle is 
directed downward, or only very slightly turned up, and the 
whole posterior region appears truncated. The rays are some- 
times very wide, dark green over a yellowish green background, 
and without the black spots. Apparently confined to the 
southern and western regions. 
8. Alasmodonta deltoidea Lea, pl. vi., fig. 2; pl. vii., fig. 4. 
Margaritana deltoidea Lea, Trans. Amer. Phil. Soc., 2 series, Vol. VI., 
p. 43, pl. xiii., fig. 38, 1836. 
Margaritana calceola Lea, Trans. Amer. Phil. Soc., 2dseries, Vol. VI., pp. 
135, 143, 148, 1838. 
Shell: Rather thick, not inflated, quadrate, compressed on 
the sides, rounded before and obtusely angular behind; dorsal 
and ventral margins straight; surface roughened by lines of 
