104 THE CHICAGO ACADEMY OF SCIENCES, 
and notched; laterals long and narrow, eleVated, striated; an- 
terior adductor muscle scar excavated, longer than wide, coarsely 
striated; posterior adductor muscle scar as long as wide, large, 
not very deeply impressed, more or less confluent; protractor 
pedis muscle scar wider than long, deeply impressed and marked 
by strong crenulations; dorsal muscle scars numerous, situated 
in the cavity of the beaks, the posterior scars the larger, all 
very deeply excavated; pallial line well marked; cavity of the 
beaks shallow; nacre white, inore or less iridescent. 
Length, 116.00; height, 63.00; breadth, 48.00 mill. $ (9295). 
“115.00; =‘ 56.00; ‘* 42.00 “ © (9294). 
t 81.00; ‘43.00; “41.00 * Gp (10860). 
“85.00; =“ 47.00; “ 8800 “ © (8058). 
“ 61.00; ‘ 8900; ‘ 26.00 “ @ (9681). 
Animal: General color whitish or cream, tentacular portion 
of mantle brownish; anal opening small, without papille; bran- 
chial opening larger with numerous papille; labial palpi large, 
cream colored, triangular, almost as broad as long, united at base 
and for ashort distance along the superior border; ctenidia large, 
the outer one the smaller, rounded, united above, color pearly 
white; foot large, white, plough shaped; liver brown; gills of 
the female darker than the male. The heart may be seen very 
plainly through the transparent walls of the pericardium; the 
pulsations are wave-like, regular and number sixteen per minute. 
The posterior part of the outer gill is used as a marsupium. 
Distribution: British America, from Great Slave Lake south 
to Texas; New York west to Montana and Dakota. 
Geological distribution: Pleistocene. 
Habitat: Found plentifully in the larger lakes and rivers, 
on a muddy bottom. In the DuPage River the specimens are 
found in a black, sticky mud, in a few feet of water. 
Remarks: This species is subject to very great variation, as 
the synonymy shows. In the sets before me, representing every 
portion of the area, there are specimens running from bright 
green, beautifully rayed, to dark brown with scarcely an indica- 
tion of rays. They vary in shape from short oval to a long 
ellipse. In thickness the variation is also great, those indi- 
viduals found in Lake Michigan being much more ponderous than 
those from the smaller lakes and rivers. This variation is due 
to the fact that Lake Michigan specimens are subject to rough 
handling by the waves while those inhabiting the smaller lakes 
and rivers are more or less protected from wave action. Speci- 
