808 PLEUROBEMA 
to the base; posterior ridge low, narrowly rounded, ending 
in a blunt point at or below the median line; anterior end 
rounded or slightly truncated; base line rounded and quite 
full at the termination of the row of nodules, from this to the 
posterior point it is straight or slightly incurved; outline of 
dorsal slope curved; surface generally sculptured with low, 
uneven, concentric ridges; epidermis tawny to brown, lighter 
colored on the dorsal slope, silky; left valve with two rather 
small, triangular pseudocardinals and two laterals; right valve 
with one to three pseudocardinals and a double lateral; beak 
cavities rather shallow; muscle scars small, impressed; nacre 
white, thicker in front. 
Length 70, height 66, diam. 38 mm. 
Ohio River; Tennessee; Claiborne, Alabama. The last local- 
ity I consider doubtful. Lea gives St. Paul, Minnesota, as a 
habitat, but I do not think it possibly can be. 
Type locality, Wabash River. 
Unio varicosus Lea, Tr. Am. Phil. Soc., [V, 1829, p. 90, pl. x1, 
fig. 20; Obs., I, 1834, p. 100, pl. x1, fig. 20—HANntey, Biv. 
Shells, 1843, p. 181, pl. xxi, fig. 14—CueEnu, Il. Conch., 
1858, pl. x1, figs. 6, 6a, 6b. 
Margarita (Unio) varicosus L&A, Syn., 1836, p. 17; 1838, p. 16. 
Margaron (Unio) varicosus LEA, Syn., 1852, p. 23; 1870, p. 34. 
Unio cicatricosus Say, N. Harm. Diss., II, No. 19, 1829, p- 
292.—Kusver, Conch. Cab. Unio, 1861, p. 185, pl. ivi, fig. 
2.—ReEVE, Conch. Icon., XVI, 1864, pls. vii, fig. 31; x11, 
fig. 50. 
Unio cicatricosus var. varicosus ParEL, Conch. Sam., III, 
T8go, p. 148. 
Pleurobema cicatricosa Stimpson, Syn., 1900, p. 765. 
Unio detectus Frierson, Naut., XXV, 1911, p. 52, pl. 11, lower ; 
pl. 111, upper figures. 
Unio cicatricoides Frierson, Naut., XXV, 1911, p. 53, pl. 11, 
upper figure. 
Not so inflated nor so large as P. esopus. The base is more 
widely rounded at the end of the row of nodules and the shell 
is higher in proportion than it is in that species. The texture 
