PILEUROBEMA 765 
from the beaks to the anterior base. his area is to a consid- 
erable degree concentrically sulcated, the sulci becoming obso- 
lete behind. where it becomes striated; epidermis rayless, dark 
reddish-brown or having faint, greenish rays near the beaks. 
Lunule triangular and membranaceous. ‘The shell is remark- 
ably flattened in front, half way from beaks to base, showing a 
sort of so-called “secondary lunule.’”’ Nacre white, to rose- 
color and iridescent. Muscle scars well impressed and sepa- 
rate. Beak cavities shallow. In the left valve there are two 
low, thick, curved laterals, somewhat striate, and a stout, up- 
right, bifid, striate, acuminate cardinal. In the right valve, 
a single low, stout, curved lateral upon a very wide, heavy 
plate or shelf and a wedge-shaped cardinal arising from a pit 
surrounded by a semicircular, low ridge. Cardinal plate thick, 
on the inner surface of which may be noted the dorsal muscle 
scars. 
Length 48, alt. 40, diam. 32.3 mm. 
Length 41, alt. 39, diam. 27.5 mm.” (Frierson). 
Type locality, Tombigbee River, Demopolis, Marengo Co., 
Ala. Also found at Columbus, Miss., and in the Alabama 
River. 
Pleurobema tombigbeanum Frierson, Naut., XXII, 1908, p. 
27, OLoMt, hes 13+ A. 
“The shell may be mistaken by the casual observer for a 
small Ouadrula pyramidata Lea, but may be easily distinguish- 
ed by its smaller size and especially its shallower beak cavities, 
lower beaks and less pronounced sulcus from beak to post- 
base. It seems to be a rather rare shell in the Tombigbee and 
Alabama rivers. Compared with P. taitianwm Lea ours is less 
convex, with a more or less distinct sulcus back of the convex- 
if ig 
PI.UROBEMA Cor (Conrad). 
Shell obliquely subtriangular, solid, inflated, inequilateral ; 
beaks very high, full and rounded; anterior end slightly trun- 
cated above, sometimes almost angled in the middle, base line 
decidedly rounded; outline of the dorsal slope curved; pos- 
