THE NATURAL HISTORY SURVEY. 61 
adult, the red color predominating in the region of the umbones; 
in some specimens the alate portion of the postero-dorsal por- 
tion is strongly wrinkled; cardinal teeth very thick, elevated, 
compressed, longer than wide, striated, two in the left and one 
in the right valve; a single female examined had a second rudi- 
mentary cardinal in the right valve; the posterior cardinal in 
the left valve is wider, thicker and more elevated than the an- 
terior cardinal; lateral teeth represented by elevated ridges; 
that in the right valve is very thick and long and fits into a de- 
pression in the left valve, the ridges on each side corresponding 
to the lateral teeth in typical Unzo; posterior adductor muscle 
scar excavated, striated, confluent, very wide; anterior adductor 
muscle scar oval, wide, not so much impressed, confluent; 
protractor pedis muscle scar very wide, impressed, striate; dorsal 
muscle scars small; very deeply excavated, situated on the pos- 
terior face of cardinal teeth; pallial line not much impressed; 
cavity of the beaks shallow; nacre silvery white with a bluish 
tinge, iridescent; in some specimens the whole en inside of 
the pallial line is a deep salmon color. 
Length, 135.00; height, 96.00; breadth, 39.00 mill. ¥ (9692). 
y's 150.00; on 12 00: 3 48.00 “' © (From Milwaukee, Wis.) 
a 108.00; “ 79.00; s PLS Uy gut d (9691). 
x 45.00; =p 40.00; os IPA juvenile (10086). 
gh 177.00; se 182.00; at 30.00 *' ov (13488). 
mg 111.00; ss 76 00; us 53.00 ‘' Y (coll. Jensen). 
u 120.00; uf 94.00; as paso: ws co (13084). 
Anima/: Generally dark flesh color or salmon; ctenidia long 
and narrow, outer crowded throughout with ova in the female, 
half-moon shaped, rounded at both ends, scalloped on the 
ventral border, chocolate brown in color, united above ajfid to 
the opposite pair for their entire length; labial palpi tether 
long, tongue shaped, color similar to the gills, united at base; 
foot large, thin, rich salmon color; the abdomen pale salmon or 
flesh color; siphons yellowish white inside, the tentacular 
portion brownish black; mantle whitish, edged with black or 
brownish; liver and cardiac pouch brownish. 
Distribution : Mississippi and Ohio drainages; Great Lakes. 
Geological distribution: Pleistocene. 
fTabitat: Found on a muddy bottom, generally in rivers, 
in water from'knee to fifteen feet in depth. 
Remarks: This is a very characteristic species, found 
generally in the larger rivers. It is at once distinguished from 
