PLEUROREMA Sor 
base of the shell; anterior end somewhat slopingly truncate 
above, rounded below; dorsal slope obliquely truncate, angled 
at the hinder end of the ligament; base line nearly straight ; 
epidermis tawny, sometimes showing rest marks, concentrically 
wrinkled and dull, rarely faintly rayed; pseudocardinals two 
in the left valve, one to three in the right; left valve with two 
laterals; right valve with a somewhat double one; beak cavi- 
ties not deep; muscle scars small, slightly impressed; nacre 
whitish or purplish tinted, thinner behind. 
Length 50, height 35, diam. 18 mm. 
‘Tennessee River system. 
Type locality, Conasauga Creek, Monroe Co., Tenn. 
Unagrbrenis Wea Pr Aco N. Sc. Phila., 11; 1872,'p. 157; Jl. 
AeeN.eoei Philas VIE, 21874; p: 35; pl- xi), fig. “32°; Obs., 
REET 7AG p> 36, pl-cxin fist.42. 
Pleurobema brezis SIMPSON, Syn., 1900, p. 763. 
This has a higher posterior ridge than P. argenteuwm, and the 
shell is usually most inflated along that line, being often wedge- 
shaped in front, when viewed from above. But there are 
intermediates that cannot be placed with certainty in either 
species. 
Var. subellipticum Simpson. 
Shell larger and more elongated and inflated than the type; 
posterior ridge decidedly double. 
Type locality, Hardy, Arkansas. 
Pleurobema brevis subelliptica Simpson, Pr. Ac. N. Sci. Phila., 
TgOO, p. 8o. 
Pleurobema brevis var. subilliptica Stmpson, Syn., 1900, p. 
763. 
This may be a valid species, but there are specimens, which 
seem to be intermediate between the two. By a typographical 
error this name was spelled subilliptica in the Synopsis. 
PLEUROBEMA MISSOURIENSE Marsh. 
“Shell smooth, obliquely triangular, rounded before, sub- 
triangular behind, moderately thick, very much thicker ante- 
riorlv, sides somewhat flattened; beaks wide, solid, incurved: 
