802 PLEUROBEMA 
ligament long, light brown; epidermis light brown, without 
rays; growth lines numerous, not raised; umbonal slope wide 
and rather flat; posterior slope wide, flattened, with two dark, 
inconspicuous lines running from beaks to posterior margin; 
beak sculpture unknown; cardinal teeth rather long and solid, 
depressed, disposed to be double in both valves, corrugate; 
lateral teeth straight, oblique, corrugate; anterior cicatrices 
distinct, deep ; posterior cicatrices distinct and well impressed ; 
shell cavity wide and deep; nacre white.” (Marsh). 
(Dimensions not given. ) 
Type locality, Black River, Poplar Bluff, Butler Co., Mo. 
Pleurobema missouriensis MArsH, Naut., XV, 1gol, p. 74. 
“T know of no described species, which this closely resem- 
bles; in outline tt is perhaps nearest to U. bigbyensis Lea, but 
differs in every other respect. Specimens of bigbyensis from 
Flint River, Ala., attain twice the size of this shell. Bigbyen- 
sis is nearly always covered with green rays. This shell is ray- 
less. It also has higher and more massive beaks, 1s more 
equilateral and differs entirely in the color of epidermis, teeth, 
etc. The color of the epidermis and the character of the beaks 
is more like U. hartmanianus Lea, but that shell has very much 
higher beaks, more swollen and pointed, and is in every respect 
a more solid shell.” 
PLEUROBEMA PLANIUS (Lea). 
Shell decidedly rhomboid, subcompressed, subsolid, inequi- 
lateral ; beaks apparently not very full or high; posterior ridge 
double, ending below in a rather wide biangulation at the base 
of the shell; above the posterior ridge there is a shallow, radial 
depression ; anterior end rounded; base line straight ; posterior 
end obliquely truncated; surface feebly concentrically sculp- 
tured; epidermis greenish-yellow, tawny behind, with a few 
broken green rays, scarcely shining, showing rest marks; left 
valve with two small pseudocardinals and two laterals; right 
valve with one pseudocardinal, a rudimentary one in front, and 
a somewhat double lateral; muscle scars rather shallow ; beak 
