UNITED STATES. 79 



Unio pallax. pi. 15, fig. 59. 



Testa Iffivi, cllipfic;i, subinflata, inasquilaterali, auticis et posticii regularitcr rotundata ; valvulis-subtenuibus ; 



natibus prominulis, aJ apicem rainutO undulata; epidermide luteo-fusca, radiatil ; dcntibus cardinalibu.s 



erectis, compressis, acuminatis, in utroque valvule duplicibus ; lateralibus subrectis sublongisque ; mar- 



garita vel alba vol pupureil vel salmonis colore tincta et valde iridescente. 



Shell smooth, elliptical, somewhat inflated, regularly rounded before and behind ; 



valves rather thin ; beaks a little prominent, minutely undulate at the tip ; epidermis 



yellowish brown, radiated ; cardinal teeth erect, compressed, sharp and double in 



both valves ; lateral teeth nearly straight and somewhat long ; nacre white, purple or 



salmon, and very iridescent. 



Proc. Aead. Nat. Sci., 1857, p. 32. 

 Hah. — Streams near Columbus, Georgia, Bishop Elliott ; and French Broad River, 

 Tennessee, Mr. Joseph Clark. 



M}' cabinet and cabinets of Bishop Elliott and Mr. Clark. 

 Diam. -7, Length 1, Breadth 1-7 inches. 



Shell smooth, elliptical, somewhat inflated, inequilateral, regularly rounded before 

 and behind; substance of the shell rather thin, very slightly thickened before; 

 beaks a little prominent, irregularly and minutely undulate at the tip ; ligament 

 rather long and thin; epidermis yellowish brown, sometimes reddish brown, with 

 rather small rays more distinctly marked on the posterior half, the lines of growth 

 being distant and strongly marked, shining on the umbones; umbonial slope rounded ; 

 cardinal teeth erect, compressed, oblique, shai'p, creuulate, and double in both valves; 

 lateral teeth long, thin, lamellar and nearly straight ; anterior cicatrices distinct, pos- 

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

 beaks ; cavity of the shell rather deep ; cavity of the beaks shallow and subangular ; 

 nacre white, purple or salmon, and very iridescent. 



Soft Parts. — Branchial uterus very large, occupying two-thirds, posteriorly, of the 

 outer branchiae, the branchial ovisacks, twelve to fifteen, being very large, extending a 

 short distance below the line of the branchiae, and having a whitish interrupted line 

 at the lower margin. In the branchial ovisacks it is analogous to heterodon, (nobis.) 

 BrancJa'crAArge, nearly semicircular, inner ones much the larger, some individuals are 

 free at the extreme point of abdominal sack, while others are closed altogether. Palpi 

 very small, very thin, subangular, united only at the upper part of the posterior edges. 

 Mantle very thin, much thickened at the margin where the edge is double, very much 

 thickened below the branchial opening, where the edge is very dark and furnished 

 with many rather distant papillne. Branchial openinf/ rather large, with very nu- 

 merous small, brownish papillae, crowded on the inner edges. Anal opening small, 

 with very minute brownish papilla) on the inner edges. S^ipcr-anal opening small, 

 colored on the edges, united for some distance below. Color of the mass whitish, 

 inclining to salmon color. 



