PLEUROBEMA 799 
ish, dull brownish or somewhat tawny, often showing rest 
marks, not shining; pseudocardinals low, ragged, two in the 
left valve and one to three in the right ; laterals two in the left 
valve and one in the right ; muscle scars impressed ; beak cavi- 
ties not deep; nacre whitish, often silvery, thinner and irides- 
cent behind. 
Length 63, height 39, diam. 20 mm. 
Upper Tennessee River drainage. 
Type locality, Holston River, Tenn. 
Ujiorarcenteus eA, Pr. Am. Phil. Soc., II, 1841, p. 82; Tr. 
Eiken Soc, .V ll 184% p. 242,pl. xxv, fig. 57; lise EE 
1842, p. 80, pl. xxv, fig. 57—CueEnu, III. Conch., 1858, pl.. 
ROME HOS, 2. Sa, 2b—Kus'rer, Ganch Cab. Unio, 1861, 
p- 188, pl. Lrx, fig. 4; 1861, p. 225, pl. Lx xvi, fig. 3—SowEr- 
BY, Conch. Icon., XVI, 1868, pl. xxxvit, fig. 204. 
Margaron (Unio) argenteus LEA, Syn., 1852, p. 26; 1870, p. 40. 
Pleurobema argentea SIMPSON, Syn., 1900, p. 763. 
The type differs somewhat from any shells I have seen, 
being more ovate in outline and having a smoother, greener 
epidermis than other shells, which Dr. Lea refers to the species. 
The shel! is larger than pyriforme and lacks the almost coppery 
luster of that form. It is not so rhomboid as flanius or 
estabrookianum and lacks the strong, concentric sulcations that 
characterize them. It is near to breve and I am not sure but 
what some of the material referred to argenteum should be 
placed in that species. The former has a higher posterior 
ridge and is more distinctly rhomboid. 
Var. pannosium Simpson. 
Shell more rude and solid than the type, the surface being 
roughly concentrically sculptured; epidermis coarse, brownish 
or tawny-brown, dull colored; nacre thick in front, suddenly 
becoming thinner. 
Length 75, height 47, diam. 26 mm. 
Type locality, Hot Springs, Arkansas. 
Pleurobema argentea pannosa Stmpson, Pr. Ac. N. Sci. Phila 
1900, p. 82; Syn., 1900, p. 763. 
