110 THE CHICAGO ACADEMY OF SCIENCES. 
muscle scar roundly ovate, scarcely impressed, confluent; Aro- 
tractor pedis muscle scar small, wider than long, deeply im- 
pressed; dorsal muscle scars minute, placed in the cavity of the 
beaks (which are large but not deep), deeply impressed; pallial 
line distinct, impressed anteriorly; nacre pearly or silvery 
white, iridescent, especially posteriorly. 
Length, 21 00; height, 12.00; breadth, 8.50 mill. ¢ (10084). 
A 21.00; oO? 2 9.00 ‘ of (10084). 
a 30.00; Ee alas os BERR. oC. Ceatons 
Animal: Generally whitish or yellowish white in color; 
ctenidia not large, rounded at both ends, united above through- 
out their entire length, pearly white in color, with a line of yel- 
lowish at their junction; the posterior extremity is thicker than 
the anterior; labial palpi rather large, whitish flecked with yel- 
lowish, triangular, united at base and partly on the posterior 
margin; anal and branchial openings rather large, edged with 
dark brown; the female is provided with a caruncle supported 
by a pedicel; mantle edged with black; foot large, pointed; ab- 
domen yellowish white; liver greenish brown; heart pulsations 
regular, thirty-six per minute; this is the largest number of.pul- 
sations for the genus. Posterior part of outer gill used as a 
marsupium. 
Distribution: Minnesota south to Texas, Western New 
York west to Iowa. 
Geological distribution: Pleistocene. 
ffabitat: Found buried in soft mud to a depth of from an 
inch to six or seven inches. It prefers slow moving streams, 
where there is a muddy bottom. 
Remarks: This is the smallest Uno found in our region. 
The female is distinguished from the male by a pronounced 
swelling in the posterior portion. It is interesting to watch this 
tiny species moving about the bottom of an aquarium; it will lie 
for a long time on the bottom, its siphons extended to their full 
length, a current of water continually flowing from the excur- 
rent siphon; if it desires to move, the long and finger like foot 
will be cautiously thrust out, extended its full length, and the 
shell pulled after it with a jerk. The umbonal sculpture is the 
coarsest of any Unio in this region, in proportion to the size of 
the shell. The young sometimes have one or two broad green 
bands, but the adult is completely rayless. The species is con- 
fined to the Desplaines River drainage. 
