PLEUROBEMA 739 
PLEUROBEMA HOLSTONENSE (Lea). 
Shell of moderate size, subtriangular, inflated, decidedly in- 
equilateral, solid; beaks very high and full, sometimes in old 
shells extending a very little in advance of the rest of the an- 
terior end; anterior end either truncate or rounded; base line 
somewhat rounded; dorsal and posterior outline curved, al- 
most angular just at the hinder part of the ligament; lunule 
small but well developed ; posterior ridge of moderate elevation, 
narrowly rounded, ending behind in a blunt, rounded point 
near the base of the shell; surface with irregular growth lines ; 
epidermis yellowish, greenish-yellow or tawny, marked with 
wide and narrow, interrupted rays; pseudocardinals rather 
small, stumpy, subradial and rough, two in the left valve and 
usually three in the right, the front and hinder one of the 
right valve vestigial; laterals two in the left valve and a dou- 
ble one in the right; beak cavities impressed; muscle scars 
small, deep; nacre whitish. 
Leneth 50, height 38, diam. 28 mm. 
Length 52, height 37, diam. 29 mm. 
Length so, height 35, diam. 25 mim. 
Tennessee River system. 
Type locality, Tuscumbia, Ala. 
Umo holstonensis Lea, Pr. Am. Phil. Soc., I, 1840, p. 288; 
ie aieel wile, SOs, Vil sto42 1p2202- pl. xy: fis 27°" Obs:, 
IIT, 1842, p. 50, pl. xv, fig. 27—Cuenu, III. Conch., 1858. 
pl. xxxu, figs. I, ta, 1b—Kuster, Conch. Cab. Unio, 1862, 
Paco ple xGvin Hes A. 
Margaron (Unio) holstonensis LEA, Syn., 1852, p. 25; 1870, 
p. 38. 
Pleurobema ho!stonensis SiMPSON, Syn., 1900, p. 746. 
Unio mundus Ima, Pr. Ac. N. Sci. Phila., I, 1857, p. 83; Jl. 
Ac. N. Sci. Phila., VI, 1866, p. 40, pl. xiv, fig. 38; Obs., XJ, 
(867, p. 44, pl. xiv, fig. 38—Rerve, Conch. Icon., XVI, 
1864, pl. xvt, fig. 72. ; 
Margaron (Unio) mundus Lea, Syn., 1870, p. 40. 
