THE NATURAL HISTORY SURVEY. 115 
Shell: Large, inflated, rather solid, inequilateral, oval; um- 
bones depressed, inflated, placed anterior to the center of the 
shell, marked by a number of coarse, rounded, even ridges, 
which cause the beaks to stand out prominently; umbonal 
slopes rounded; dorsal margin slightly arched, ventral border 
rounded; anterior and posterior margins rounded or subtrun- 
cated; surface shining, marked by very strong lines of growth; 
color yellowish or brownish, sometimes dark horn color; liga- 
ment weak, dark horn color; cardinal teeth very small, a single, 
elevated arched tooth situated just beneath the beak, the two 
arches of unequal length, in the right valve, and two narrow, 
solid, elevated, pyramidal teeth which are placed asin solidulum, 
the tooth near the ventral border being smaller and more ele- 
vated, in the left valve; the hinge line is a trifle thickened; 
lateral teeth on each side of the cardinals short, thick, solid, 
elevated, serrated, double in the right and single in the left 
valve; muscle scars and pallial line faint; nacre bluish white, 
with zones of very dark purple or horn color. 
Length, 14.00; height, 11.00; breadth, 9.00 mill. (9885). 
OS Te.a0e - Pe ieee meal arg) 8) BETO). 
the TOGO: 8) B.00F ce fF gg “8 . | (9882): 
mm agg? he Sg O0-m) AP Sao. (S81). 
Anima/; Bluish or whitish, transparent, yellowish about the 
base of the siphons and brownish on the dorsal part near the 
liver and heart; mantle lobes thin and transparent, edged with 
brownish yellow or yellowish white, very muscular; siphons 
long, united only at base, ventral very large and longest, not 
ciliated; adductor muscles not large and rather weak; foot re- 
tractor and protractor muscle well developed; foot long and 
digitiform, transparent, and capable of extension more than the 
length of the shell; it is thrust from a cleft in the mantle about 
the center of the ventral border of the shell; ctenidia large, the 
outer the smallest, rounded before and behind, united above; 
labial palpi long and narrow, ciliated, united at base, bluish 
white in color; pericardium situated well on the dorsal surface; 
heart very plainly seen, beats regular, fifty-seven per minute. 
Distribution: New England west to Arkansas and Iowa, 
Illinois south to Alabama and Louisiana. 
Geological distribution: Pleistocene. 
Hlabitat: In small creeks, ponds and rivers, buried in black, 
slimy mud to a depth of several inches. 
