UNITED STATES. 83 



Femarlcs.—Two specimens only were received from Bishop Elliott. It is a species 

 allied to angmtatm, but not quite so wide. It has some resemblance to the young of 

 HallenhechU, (nobis.) The posterior slope is carinate. The basal margin is disposed 

 to be emarginate being compressed in the middle. The rays cover the whole disk, 

 in these specimens, very beautifully, although they are very dark. Both the speci- 

 mens before me are purple in the nacre. Other specimens may be white or salmon, 

 but I suspect that purple will predominate. 



Unio tenebricus. pi. 17, fig: 63. 



Testa Incvi, clliptiou, subiuflatfi, po.'ticc obtuse angulata, in.xquilaterali, valvulis tcnuibus, antico crassiori- 

 bu3 ; natibus prominulis ; epidermide tenebroso-fusca, obsolete radiata ; dentibus cardinalibus parvis, 

 valde crcnulatis, in utroque valvule duplicibus ; lateralibus lamellatis curvisque ; margarita vel pur- 

 purea vel salmonis colore tincta et iridescente. 



Shell smooth, elliptical, slightly inflated, obtusely angular behind, inequilateral ; 

 valves thin, thicker before; beaks slightly prominent; epidermis dark brown and obso- 

 letcly radiated ; cardinal teeth small, very much crenulate, double in both valves ; 

 lateral teeth lamellar and curved ; nacre purple or salmon color and iridescent. 

 Proc. Acad. Nat. Sci., 1857, p. 171. 



Hah. — Etowah River, Georgia. Bishop Elliott and Rev, Geo. White. 



M}' cabinet and cabinets of Bishop Elliott, Mr. White and Mr. Bland. 

 Diam. -6, Length 1-1, Breadth 1-9 inches. 



Shell smooth, elliptical, slightly inflated, obtusely angular behind and round before, 

 inequilateral ; substance of the shell thin, thicker before ; beaks a little prominent, 

 ligament rather short and thin ; epidermis dark brown, obsoletely radiated, with rather 

 distant marks of growth ; cardinal teeth small, erect, pointed, very much crenulated 

 and double in both valves ; lateral teeth rather long, lamellar and curved : anterior 

 cicatrices distinct and well impressed ; posterior cicatrices confluent and very slightly 

 impressed ; dorsal cicatrices placed in the centre of the cavity of the beaks ; cavity 

 of the shell shallow and wide ; cavity^ of the beaks rather shallow and rounded ; 

 nacre purple or salmon color and iridescent. 



Soft Parts. — Branchial titerm . Ova in ovarium, but none in the branchial ovi- 



sacks. Branchia; very large, nearly semicircular, inner ones very much the larger, 

 not free. Palpi small, thin, suboval, united only at the upper end of the posterior 

 edges. Mantle thin, thickened at the margin and colored on the edges. Branchial 

 opening rather large, with numerous dark brown papillse on the inner edges. Anal 

 opening small, with very small brown papilla on the inner edges. Super-anal open- 

 ing rather small, colored on the edges and united below. Color of the mass light 

 salmon. 



Remarlv. — I have seen quite a number of this species. It belongs to that group 

 of which )ii/e)i<> (nobis) may be considered the typo. It is also allied to Vanu.remii 



