35 



affinity than with any other species. It appears to 

 be destitute of rays. 



This is the only Unio of a suborbicular form yet 

 discovered in the Atlantic rivers. I have only two 

 specimens, the largest of which is figured. 



I have given this shell the name of my kind friend, 

 Mr. William Mason, one of the earliest American 

 Conchologists, whose discoveries have enriched our 

 cabinets with many new and rare species of land and 

 fresh water shells. 



UNIO DELUMBIS. 



Plate Y.—Fig. 3. 



DESCRIPTION. 



Shell ovate, elongated, very thin and fragile, ven- 

 tricose; ligament margin slightly elevated; anterior 

 side rather narrow, posterior margin rounded; epi- 

 dermis olivaceous, with green rays; umbonial slope 

 rounded; margin of the ligament slope rounded, very 

 oblique; within bluish, higJily iridescent; cardinal 

 teeth laminar; cavity capacious. 



OBSERVATIONS. 



I found this shell abundantly near Cooper river, 

 South Carolina, in the small streams; it is more 

 nearly allied to U. ochraceus, of Say, than to any 

 other species; but it may be distinguished by its elon- 



