56 THE CHICAGO ACADEMY OF SCIENCES. 
Shell: Very thin and fragile, more or less elliptical, some- 
what inflated, broadly rounded before and triangular behind, 
where it is much produced; dorsal margin obsoletely straight, 
ventral margin slightly rounded; surface smooth and shining, 
lines of growth very faint, excepting on the alate portion of the 
postero-dorsal margin and at the posterior end; here and there 
there is a black line which marks a heavier growth line than 
usual; umbones perfectly flush with the hinge line, brownish 
horn in color and marked by several concentrically arranged 
ridges of small size; the umbones are placed pretty well toward 
the anterior end; posterior slope subexcavated, anterior slope 
rounded; ligament thin and weak, light brown in color; epi- 
dermis bright grass green, brownish horn about the umbones, 
with numerous darker green rays extending to the ventral mar- 
gin; hinge line simple and thin; muscle scars and pallial line 
rather faint; beaks without cavity; nacre silvery white, very 
iridescent. 
Length, 59.00; height, 29.00; breadth, 21.00 mill. (12454). 
“50.00; ‘ 26.00; “ 18.00 " (12454). 
i 41.00; + 2200s " 12.00 ‘* (12454). (Young.) 
Animal: Anal opening small, outer rather large, with many 
brown papille; branchie slightly curved below, nearly equal in 
size, free nearly the whole length of the abdominal sac, marsu- 
pium occupying the whole of the outer gill. (Simpson.) 
Distribution: Western New York to Arkansas, Canada to 
Texas and Louisiana. 
Geological distribution: Pleistocene. 
Habitat: Found on a muddy bottom in creeks, small rivers 
and ponds. 
Remarks: This is one of the most beautiful and one of the 
most distinct of all Anodontas. Its beautiful grass green, fragile 
shell, so delicately rayed, its umbones flush with the hinge line 
and its peculiar shape at once distinguish it from all other forms. 
The young are frequently somewhat alate and some specimens 
are lighter in color than others, and they may be rayed or ray- 
less. The only locality at present known is at Willow Springs, 
on the Desplaines River, where it is said to be quite abundant. 
Genus ALASMODONTA Say, 1819. 
Shell: Solid rhomboid with a posterior ridge more or less 
biangulate behind, with a smooth, hard, shining epidermis, which 
is usually rayed; beak sculpture consisting of a few coarse 
