42 



lonreJ in the nacre, and are smooth and pohshed on 

 the exterior. The figure represents a (specimen 

 nearly of the largest size which I obtained. 



U N rj T E N E B R O S U S . 

 Plate YU.—Fig. 1. 



DESCRIPTION. 



Shell elliptical, ventricose; valves rather thick ante- 

 riorly; beaks prominent; posterior margin descending 

 obhque and rectilinear i'rom the ligament: extremity 

 subtruncated; epidermis oliv^aceous; 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. ochraceiis and cariosiis, Say, precisely hke 

 those inhabiting the Schuylkill river, near Philadel- 

 phia. 



