42 
loured in the nacre, and are smooth and polished on 
the exterior. The figure represents a specimen 
nearly of the largest size which I obtained. 
UNIO TENEBROSUS. 
Plate VII. — Fig. 1. 
DESCRIPTION. 
Shell elliptical, ventricose; valves rather thick ante- 
riorly; beaks prominent; posterior margin descending 
oblique and rectilinear from the ligament; extremity 
subtruncated; epidermis olivaceous; rayed; margins 
dark brown; cardinal teeth moderately thick, oblique; 
within bluish or pink; iridescent; cavity capacious. 
OBSERVATIONS. 
Two specimens of this species were sent to Mr. 
Poulson by Mr. Featherstonhaugh, who found them 
in the Rappahannock, near Falmouth, Stafford 
county, Virginia; I also found a single specimen in 
South Carolina. On comparing these with the U. 
radiatus, Barnes, I find they differ from that species 
in their greater convexity, and in the somewhat 
pointed posterior side; they are also more obscurely 
rayed. Those of the Rappahannock were associated 
with U- ochracens and cariosus , Say, precisely like 
those inhabiting the Schuylkill river, near Philadel- 
phia. 
