764 PLEUROBEMA 
PLEUROBEMA TAITIANUM (Lea). 
Shell obliquely triangular, solid, inflated; beaks very full 
and high, placed at or in advance of the anterior end of the 
shell; anterior end obliquely truncated, sometimes showing an 
almost double lunule, rounded below ; base line straight ; post- 
dorsal line only slightly curved; posterior ridge low, ending in 
a rounded point near the base of the shell; in front of the 
posterior ridge there is a wide, radial swelling; surface with 
uneven, concentric ridges; epidermis tawny or tawny-brown, 
scarcely shining; pseudocardinals broken up, solid, ragged, 
two in the left valve and one, with often one or more small 
ones, in the right; laterals irregular, two in the left and a dou- 
ble one in the right; muscle scars small, deep; beak cavities 
shallow ; nacre white or pink, thin and iridescent behind. 
Length of type 40, height 40, from beak to post-basal point 
45, diam. 26 mm. 
Length 52, height 48, diam. 24 mm. 
Type locality, Alabama River. 
Unio taitianus Lea, Tr. Am. Phil. Soc., V, 1834, p. 39, pl. rv, 
hg. 11; Obs., I, 1834, p. 151, pl. rv, fig. 11.—Hantey, Biv. 
shells, 1243, po 186, plaxxann fie. 26. 
Margarita (Unio) taitianus Lea, Syn., 1836, p. 21; 1838, p. 18. 
Margaron (Unio) taitianus Lea, Syn., 1852, p. 25; 1870, p. 38. 
Pleurobema taitiana SIMPSON, Syn., 1900, p. 754. 
This shell bears a most striking superficial resemblance in 
shape to Quadrula pyramidata, but the epidermis is very differ- 
ently colored and the beak cavities are unusually shallow, while 
in QO. pyramidata they are deep and compressed. A specimen 
from Claiborne, Alabama, in the Lea collection has rich pink 
nacre. 
PLEUROBEMA TOMBIGREANUM Frierson. 
“Shell short, triangular, thick, solid and heavy; truncated 
in front, roundly pointed behind at the post-base. Beaks high 
and incurved, their sculpture not seen. Post-ridge rounded 
and close to post-margin. The sides are slightly flattened just 
in front of the post-ridge and an inflated, raised area extends 
