98 THE CHICAGO ACADEMY OF SCIENCES, 
small, rounded pits, followed by a single long and narrow scar 
very deeply impressed; cavity of the beak wide, shallow; nacre 
varying from deep purple, through mauve to salmon pink, iri- 
descent. 
Length, 114.00; height, 74.00; breadth, 36.00 mill. 
uu LOO ot 92700 37.00 « 
G SSL00 es: O00 me 19:00. ** 
Animal: Anal opening small, without papille; branchial 
opening small with vestiges of papilla; palpi subtriangular, 
united only slightly behind; inner gill united throughout to 
abdominal sac; branchial uterus occupying posterior part of 
the outer gill, very distinct, projecting below inner gill. 
(Simpson). a 
Distribution: Western New York west to Nebraska and 
Iowa, Wisconsin south to New Mexico. 
Geological distribution: Pleistocene. 
Habitat; In the larger lakes and rivers, on a muddy bot- 
tom, in water from five to twenty feet in depth. 
Remarks: Alatus 13 avery distinct species, easily known by its 
rich purple interior and very alate postero-dorsal margin. The lat- 
ter is more pronounced in young than in old specimens, the lat- 
ter becoming more oval in form and losing to some degree their 
pronounced alate character. The division between the posterior 
umbonal slope and the alate “ wing” is marked by an area of the 
lines of growth which is raised in strong, wavy ridges. The rays 
vary to some extent, the young shell having rather wide green, 
rays on a yellowish background; half-grown specimens have 
narrow rays on a yellowish green background, while old speci- 
mens have a perfectly rayless epidermis. The cardinal teeth 
also vary in some specimens, being rather long and narrow, with 
sulcations on the anterior faces, while in others they are more 
solid, wider, long, triangular, and the sulcations are extended 
to the upper surface of the teeth, which are all directed toward 
the antero-ventral border of the shell. The anterior adductor 
muscle scar almost always has a heavy ridge between it and the 
protractor pedis scar, the ridge bearing a portion of the adductor 
scar. The alate portion of the shell extends far above the lateral 
teeth. This species has been found in the southern region in 
the Calumet River, and in the western region in the Desplaines 
River. It does not appear to be common. 
