THE NATURAL HISTORY SURVEY. 73 
teriorly in the female); elliptical, rounded before and obtusely 
angled, or rounded behind; dorsal margin straight, ventral mar- 
gin rounded; surface marked by rather coarse lines of growth; 
umbones a trifle elevated, inflated, greenish gold in color when 
not eroded, and marked by three or four small, but well defined 
ridges directed anteriorly; anterior umbonal slope short and» 
rounded; posterior slope long and somewhat flattened; ligament 
weak, light brown in color; epidermis light green, yellowish 
green or brownish, with numerous wide, dark green rays, 
extending from the umbones to the ventral margin; the umbones 
are not rayed; there is generally a dark brown line near the 
ventral border in the young, marking an old rest period; hinge 
line straight, feeble, only very slightly thickened, the cardinal 
teeth but slightly developed; muscle scars and pallial line very 
faint; cavity of the beaks shallow; nacre bluish white, iridescent. 
Length, 56.00; height, 32.50; repens 21.00 mill. ¢ (9841). 
i 55.00; ‘* 29.00; 21.00 ‘ Gy (9841). 
< eee APP? CEOIGU,® e84 (o 27.00 ‘ of (8422). 
3 81.00; ‘* 42.00; ch 40.00 ‘* ® (coll. Jensen). 
‘s 74,00; 88,00; ‘ R00 4 Gh" a 
Animal; Generally light flesh color or white and transparent; 
ctenidia very wide but not very long, yellowish white, rounded 
at both ends, united above throughout their entire length; right 
and left pair not entirely united, leaving a space about half an 
inch, where they hang free; all four bronchiz filled with ovules 
in the female; labial palpi wide, large, tongue shaped, dark 
flesh color with a patch of yellowish white on the dorsal margin, 
united at base; siphons thick, papillose variegated with cinna- 
mon brown, the extreme border edged with black; mantle white 
and transparent, edged with black; liver dark brown; abdomen 
pearly white; foot large and muscular; heart pulsation regular, 
sixteen per minute. 
Distribution: Mississippi River drainage and St. Lawrence 
drainage. New York west to Dakota, Southern Canada south 
to Texas. 
Geological distribution: Pleistocene. 
Habitat: Found commonly in lakes and rivers, on a muddy 
bottom. 
Remarks: Distinguished from all other species found in 
this region by its grass green color, minute umbonal sculpture 
and delicate rays. It is found associated with SS. edentulus, A. 
grandis, A. pressus and L. luteolus, all of which are mud loving 
