THE NATURAL HISTORY SURVEY. 109 
branchiz very large, nearly circular below, inner larger, united 
to abdominal sac or free; marsupium occupying two-thirds of 
the posterior part of outer branchiz, greatly distended. 
(Simpson. ) 
Distribution: New York to Dakota and Kansas, Michigan 
and Minnesota to Louisiana and Alabama. 
Geological distribution: Pleistocene. 
Habitat: Found in muddy and sluggish rivers, in soft mud. 
Remarks: This is a variable species which is frequently con- 
founded with /uteolus. It isa more elliptical shell in general, 
but the ornamentation of the umbones shows the greatest differ- 
ence; in /uteolus they are of good size, numerous and wavy, 
while in /¢gamentinus they are small and fewin number. The 
nacre is very variable, passing from pure white, through bluish 
white, to pinkish. The epidermis of the present species varies 
somewhat, some specimens being yellowish with wide dark 
green rays, while other specimens are almost black, the rays 
being represented only by darker bands. When at its best, it is 
one of our most beautiful shells. It seems to be confined to 
the Desplaines River drainage. 
Section CORUNCULINA Simpson. 
GROUP OF LAMPSILIS PARVUS 
36. Lampsilis parvus Barnes, pl. xiii., fig. 3. 
Unio parvus Barnes, Amer. Jour. Sci. and Arts, Ist series, Vol. VI., p. 
274, fig. 18, 1823. 
Shell; Small, thin, inflated, elliptical, rounded before and 
behind; dorsal border straight, ventral border very slightly 
rounded; surface roughened by coarse lines of growth which 
are more or less elevated; umbones large for the size of the 
shell, not elevated, inflated, yellowish brown in color, marked 
by five or six elevated, strong, parallel curved ridges, which are 
nearer the hinge line posteriorly than anteriorly; anterior um- 
bonal slope rounded, posterior slope rounded or with a slight 
angle, excavated near the beaks; ligament long, narrow, fragile, 
dark horn color; epidermis blackish or olive green, sometimes 
with a golden luster, rayless; cardinal teeth small, single in the 
right and double in the left valve, erect, thin, acuminate, ser- 
rated, widely separated; there is sometimes a second very small 
tooth in the right valve, near the hinge line; lateral teeth long, 
slightly striate, smooth, lamellar; anterior adductor muscle 
scar forming a truncated oval, impressed; posterior adductor 
