806 PLEUROBEMA 
post-basal part of the valves; epidermis tawny-brown in the 
younger shell, becoming darker with age; hinge plate solid, 
not flattened; pseudocardinals triangular, rough; cavity of the 
beaks not deep; front part of the shell very heavy ; hinder part 
much thinner. 
Animal having the mantle somewhat toothed posteriorly ; 
branchial and anal openings large, the latter smooth or with 
very faint papilla; inner gills the larger, rounded below, free 
nearly or quite their whole length from the abdominal sac ; 
marsupium occupying the entire outer gills. 
Type, Unio esopus Green. 
Ortmann, (1. c.), raises this group to generic rank. 
PLEUROBEMA AHSOPUS (Green). 
Shell irregularly oval, inflated, solid, inequilateral; beaks 
full and high, turned slightly forward over a lunule; their 
sculpture a few irregular ridges: anterior end rounded; front 
half of base rounded; at about the middle of the shell a row of 
irregular knobs runs from the beaks to the base; behind this 
row of knobs the base line is straight or incurved; outline of 
the dorsal slope curved; posterior ridge low, rounded, ending 
in a blunt point about on the median line ; surface with uneven, 
concentric sculpture; epidermis straw-color and shining in 
young shells, becoming brown and roughened in old shells; 
left valve with two strong, triangular pseudocardinals and two 
stout laterals; right valve with one to three pseudocardinals 
and a more or less double lateral; muscle scars nearly round, 
impressed; beak cavities rather shallow; nacre white, thicker 
in front. 
Length 120, height 80, diam. 55 mm. 
Ohio, Cumberland and Tennessee river systems; west to 
Missouri and Minnesota; Alabama River, according to Call. 
Type locality, Pittsburgh, Pa. 
Unio wsopus GREEN, Cont. Mac. Lyceum, I, No. 2, 1827, p. 46, 
fg. 3.—HAaniry, Biv. Shells, 1843, p. 181, pl. xxtv, fig. 7. 
—Catt, Jl. Cin. Soc. N. H., XVIII, 1896, p. 157, pl. vr. 
Margarita (Unio) wsopus Lea, Syn., 1836, p. 17; 1838, p. 16. 
