200 DESCRIPTION OF 



of the beaks; cavity of the shell deep and rounded; cavity of the beaks small; 

 nacre very white and iridescent at the posterior portion. 



Remarhs. — This is remarkable for its perfectly elliptical margin, its white 

 nacre and roundness of the umbonial slope. It has somewhat the aspect of U. 

 stramineus (Conrad) but differs from it in being a more perfect ellipse, in 

 liaving less polish, and in the wrinkles being larger and more regular. It was 

 taken by Mr. Edgar in Stone's River near Nashville, and I owe to that gentle- 

 man's kindness the specimen described. — The beaks in this are eroded, but they 

 are perfect enough to present an appearance of radiate folds diverging from 

 the tip of the beaks, like U. delodontus Lam. {lacleolus, Lea, Trans. Am. Phil. 

 Soc. Vol. 5, page 8, Fig. 19,) and other South American species. Should 

 these folds be found really to exist, when perfect specimens are had, this spe- 

 cies will exhibit the only case yet observed among our JJniones with that 

 character which is so common to those of South America. 



Unto amcenus. Plate 10, Fig. 12. 



Testa elliptica, suhcompressd, valde incequilaterali ; valvuUs subcrassis ; natibus subprominentibus ; 

 epidermide tenebrosd, dense radiatd; dentibus cardinalibus parvis, erectisque ; lateralibus longis sub- 

 curvisque; margarita alba et iridescente. 



Shell elliptical, soijiewhat compressed, very inequilateral; valves rather thin ; beaks somewhat prominent; 

 epidermis dark, very much radiate; cardinal teeth small and erect; lateral teeth long and curved; nacre 

 white and iridescent. 



Hab. Holston River, Tenn. S. M. Edgar. 



My Cabinet. 

 Diam. .7, Length 1.1, Breadth 1.9 inches. 



Shell elliptical, somewhat compressed, very inequilateral, sub-emarginate at 

 base; substance of the shell rather thick, thinner behind; beaks somewhat promi- 

 nent; ligament rather long and thin ; epidermis dark, with numerous green rays 

 over nearly the whole disk; cardinal teeth small, double in both valves, erect 

 and pointed; lateral teeth long and curved; anterior cicatrices distinct; posterior 

 cicatrices confluent; dorsal cicatrices placed in the centre of the cavity of the 

 beaks; cavity of the shell shallow; cavity of the beaks shallow and angular; 

 nacre very pearly and iridescent. 



