60 THE CHICAGO ACADEMY OF SCIENCES. 
Distribution: New York to Iowa, Michigan to Kentucky. 
Geological distribution: Pleistocene. 
Habitat: In creeks, buried in black, slimy mud to the 
depth of five or six inches. In this area it is found only in 
creeks, never in rivers or ponds. 
Remarks: This species strongly resembles A/asmodonta 
rugosa Barnes, and is taken for that species by very many con- 
chologists. In pressa the posterior margin is seldom rough- 
ened and the lateral teeth are generally more developed than in 
rugosa; the purple band on the edge of the valve in rugosa is absent 
in pressa. The characters of the cardinal teeth are very peculiar, 
and will at once distinguish this species. Specimens from 
Hickory Creek, where the species is abundant, have a yellowish 
epidermis beautifully rayed with deep grass green, and the um- 
bones are distinctly marked. Just beneath the cardinal teeth, 
and posterior to the anterior muscle scar, the shell is thickened 
by a heavy deposit of shelly matter. 
GROUP OF ALASMODONTA COMPLANATA. 
6. Alasmodonta complanata Barnes, pl. viii., figs. 1, 2; pl. ix., figs. 1, 
2, 8, 4. 
Alasmodonta complanata BARNES, Amer. Jour. Sci. & Arts, 1st ed., Vol. VI, 
p. 278, pl. xiii., figs. 17a, 17b, 1823 (preoccupied by Unzo complanatus 
Solander). 
Margaritana katherine Lea, Synop. Fam. Naid., 2d ed., 1839. 
Shell: Very large and thick, heavy, subquadrate, alate, espe- 
cially in the young; compressed in the male and inflated in the 
female; rounded before and triangulate behind; the angles 
straight; in the young shell this division is not so pronounced, 
the posterior border being broadly rounded; dorsal margin but 
slightly curved, ventral margin with a gentle curve in the male 
and a pronounced curve in the female; umbonal slopes almost 
flat in the male but strongly rounded in the female, the posterior 
slope a trifle excavated; surface marked by coarse lines of 
growth, which are generally raised into sharp ridges; umbones 
not elevated or inflated, light yellowish in color in the young, 
but changing into brown and black in old shells, and marked by 
four coarse, elevated ridges arranged in a double loop, the apex 
directed anteriorly; the umbones are frequently eroded; ligament 
very strong, long, narrow, dark brown or black in color; epi- 
dermis variable, yellowish brown marked by darker rays in the 
very young, changing to black or greenish, slightly rayed in the 
half grown forms, and deep reddish black or jet black in the 
