PLEUROBEMA 
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Group of Pleurobema swordianum. 
Shell oval, convex; beaks only moderately elevated, epider- 
mis nearly or quite ravless; teeth strong; laterals reaching well 
forward; muscle scars large, deep; nacre lurid. 
PLEUROBEMA SWORDIANUM (S. H. Wright). 
Shell large, irregularly ovate, not inflated, quite inequilat- 
eral; beaks moderately elevated and full, placed well forward; 
lunule narrow ; ligament long; posterior ridge widely rounded: 
outline of dorsal slope almost evenly curved; posterior end 
rounded ; base line slightly curved; anterior end rounded ; sur- 
face widely, concentrically striate; epidermis tawny-brownish, 
nearly or quite ravless; left valve with two strong, rough, 
radial pseudocardinals and two strong, curved laterals; right 
valve with three radial pseudocardinals and a somewhat double 
lateral; laterals reaching forward to the pseudocardinals ; 
beak cavities shallow; muscle scars latge, deep; pallial line 
crenate, remote from the border; nacre lurid, greenish-vellow. 
Length 95, height 71, diam. 38 mm. 
Type locality, Powell’s Creek, Lee Co., Va. 
Unio swordianus 5. H. Wricut, Naut., XI., 1897, p. 4. 
Pleurobema swordiana Simpson, Pr. Ac. N. Sci. Phila., 1900, 
p. 81, pl. iv, fig. 4; Syn., 1900, p. 750. 
A peculiar and puzzling form, which has characters of sev- 
eral groups. It has something of the form of Quadrula kirt- 
landiana, and it resembles O. bursa-pastoris, but it has shallow 
beak cavities. It has some of the characters of P. appressiwm, 
but is larger, has large muscle scars, remote pallial line, and 
it has remarkably elongated laterals, which reach forward to 
near the front part of the pseudocardinals. J have placed it 
for these reasons in a separate group. 
Group of Pleurobema decisum. 
Shell solid, inflated, ovate to elliptical, very inequilateral, 
somewhat truncated in front and rounded or bluntly pointed 
behind, wedge-shaped when looked at from above ; base slight- 
