796 PLEUROBEMA 
Margaron (Unio) striatus Lea, Syn., 1852, p. 26; 1870, p. 40. 
Pleurobema striata SIMPSON, Syn., 1900, p. 762. 
A small. rather thin, compressed species, having the dull 
epidermis decidedly, concentrically wrinkled, so that the un- 
rubbed shell is cloth-like. There is a good deal of variation in 
the development of the posterior ridge. 
PLEUROBEMA GIBBERUM (Lea). ° 
Shell subrhomboid, convex, rather solid, inequilateral ; beaks 
moderately elevated and full; posterior ridge double, ending 
at the base of the shell; anterior end a little narrowed, round- 
ed: base line nearly or quite straight; outline of dorsal slope 
lightly curved; surface with delicate, uneven, concentric sculp- 
ture; epidermis reddish-brown, subshining; left valve with 
two pseudocardinals and two laterals; right valve with three 
pseudocardinals and one lateral; muscle scars not deep; beak 
cavities shallow ; nacre bronzy or coppery-purple, thin and iri- 
descent behind. 
Length 45, height 26, diam. 14 mm. 
Type locality, Caney Fork River, Tennessee. 
Unio gibber Lea, Tr. Am. Phil. Soc., VI, 1838, p. 34, pl. x, 
fig. 30; Obs., II, 1838, p. 35, pl. x, fig. 30—HANLEy, Biv. 
Shells, 1843, p. 185, pl. xx1, fig. 46—CueENu, Il. Conch., 
1858, pl. Xxi1!, figs. 2, 2a, 2b.--Sowerpy, Conch. Icon., XVI. 
1866, pl XE, fey 210! 
Margarita (Unio) gibber Laas, Syn., 1836, p. 20; 1838, p. 17. 
Margaron (Unio) gtbber Lea, Syn., 1852, p. 24; 1870, p. 37- 
Pleurobema gibber Stimpson, Syn., 1900, p. 762. 
Close to pyriforme and modicum. Smaller and more trt- 
angular in outline than the former: larger, less inflated and 
having a more distinctly biangulate posterior ridge than the 
latter. 
PLEUROBEMA RAVENELIANUM (lea). 
Shell -subovate or subrhomboid, subinflated, inequilateral, 
solid; anterior end rounded; base lightly curved or straight ; 
post-dorsal line curved or obliquely subtruncated ; beaks full, 
moderately high; posterior ridge well developed, rounded, 
