758 PIL.EUROBEMA 
ly angled in front, then nearly straight for two-thirds of its 
length, from whence it curves to the posterior point; beaks 
high, curved inward and forward; beak sculpture consisting 
of a few coarse, irregularly concentric ridges, which curve 
slightly upward behind; epidermis tawny to brownish, rayless, 
the rest periods very distinctly marked by dark bands; pseudo- 
cardinals stumpy, ragged, often showing a tendency to elonga- 
tion in the direction of the axis of the shell. 
Animal having the branchiz rather small, inner the larger, 
free nearly or quite the entire length of the abdominal sac, 
marsupium occupying all but the extreme posterior end of the 
outer gills; branchiz and anal openings papillose. 
PLEUROBEMA DECISUM (Lea). 
Shell somewhat elongated, subtriangular, solid, inflated; 
beaks high and full, placed close to the anterior end in young 
shells, and projecting well in advance in the old ones; lunule 
extending under the beaks; anterior end sometimes rounded 
in young shells, sharply cut away below in old ones; base line 
rounded; outline of the dorsal slope slightly curved or even 
straight in old shells; posterior ridge not greatly elevated, 
rounded or subangular, ending in a rounded point about at 
the median line. In front of the posterior ridge there is a 
wide, radial swelling that is much fuller than it is; epidermis 
tawny to greenish-brown or brown, nearly or quite rayless, 
often showing the dark rest marks very plainly, there are fre- 
quently one or two broad, dark, dimly outlined rays at the 
posterior part of the shell; pseudocardinals irregular, radial, 
two in the left valve and three in the right; left valve with 
two laterals ; right valve with a double one ; muscle scars small, 
the anterior deep, the posterior round and impressed; beak 
cavities shallow ; nacre silvery, thicker along the anterior base. 
Length 8o, height 40, diam. 34 mm. 
Alabama and Tombigbee River systems. 
Type locality, Alabama River. 
