PLEUROBEMA 803 
cavities not deep; nacre whitish, purple tinted and iridescent 
behind. 
Length 58, height 38, diam. 19 mm. 
Upper Tennessee River drainage. 
Type locality, Tennessee; Holston River, Washington Co., 
Va. 
Unio planior Lea, Pr. Ac. N. Sci. Phila., XII, 1868, p. 145; 
jieekes Neeser Phila Vl S60) p: 316; pl 1, fig: 120° Obs., 
NIT, 1869, p. 76, pl. @ fig. 129. 
Margaren (Unio) planor Lia, Syn., 1870, p. 35. 
Pleurobema planior SIMPSON, Syn., 1900, p. 763. 
The only shell I have seen, which I am certain is this, is the 
type, which is probably young. Compared with specimens of 
estabrookianum of equal size it is more elongated, straighter 
on the base, less sulcate and has broken rays. It is less solid 
and more compressed than P. breve or its variety. 
PLEUROBEMA ESTABROOKIANUM (Lea). 
Shell rather large, subrhomboid, compressed, solid, some- 
what inequilateral ; beaks compressed but considerably elevated, 
slightly turned forward over a narrow lunule, their sculpture 
a few strong, irregular ridges; posterior ridge more or less 
double, curved, ending near the base in a feeble biangulation ; 
above them there is a light, radial depression; anterior end 
rounded; base line more or less curved; outline of dorsal slope 
almost evenly curved in old shells, angular at the middle in 
young ones; surface strongly, concentrically sculptured; epi- 
dermis straw-colored in young shells, tawny or greenish-brown 
in old: ones, showing rest marks; pseudocardinals strong, two 
in the left valve, two or three in the right; left valve with two 
laterals; right valve with one, which is sometimes double; 
muscle scars well impressed; beak cavities shallow; nacre sil- 
very white and thinner behind, thicker and duller colored in 
front. 
Length 78, height 57, diam. 24 mm. 
Upper Tennessee River drainage. 
Type locality, Clinch River; Second Creek (Knoxville ?), 
Tenn. 
