UNITED STATES. l\i 



former, but its rays are more like the latter, which is less inflated. Tlie teeth are 

 more like those of concavu-s than those of Prevastiann.'i. The substance of the shell 

 is about as thick as the former. Its bright and yellow epidermis makes a fine con- 

 trast to the beautiful green interrujited rays, which cover the whole disk. The figure 

 is from a male. I have two females in alcohol, both of which have the postei'ior part 

 of the shell very much enlarged and rounded on the umbonial slope. The papilla; 

 on the margin extend from the brancfilal npeirin;/ quite half way of the basal margin 

 forming a remarkably fine fringe. 



Unio tetricus. pi. 22, fig. 78. 



Testa laevi, oblonga, vald6 compressa, postice biangulata, inequilateral!; valvulis crassiuscuHs ; natibus 

 prorainulis ; epidermide rugoso-striata, tenebroso-f'usca, obsolete radiata ; dcntibus cardiiialibus parvis, 

 conicis, crenulatis, in utro(|UC valvulo subdujiliuibus; lateralibus subloiifris subcurvisi|ue; iiiargarita 

 purpurascens et iridesceiite. 



Shell smooth, oblong, very much compressed, biangular behind, inequilateral ; 

 valves a little thickened; beaks a little prominent; epidermis roughly striate, dark 

 brown and obscurely radiate ; cardinal teeth small, conical, crenulate, and in both 

 valves somewhat double ; lateral teeth rather long and somewhat curved ; nacre 

 purple and iridescent. 



Proc. Acad. Nat. Sci., 1857, p. 170. 



Bab. — Flint River, near Albany, Georgia. Bishop Elliott. 



My cabinet and cabinet of Bishop Elliott. 

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



Shell smooth, oblong, very much compressed, biangular behind, inequilateral; 

 substance of the shell a little thickened ; beaks slightly prominent ; ligament rather 

 long and thin ; epidermis roughly striate, very dark brown and very obscurely rayed ; 

 umbonial slope slightly raised and obtusely angular ; cardinal teeth small, conical, 

 somewhat erect, crenulate and disposed to be double in the right as it is in the left 

 valve ; lateral teeth rather long and somewhat curved ; anterior cicatrices distinct and 

 well impressed ; posterior cicatrices large and confluent ; dorsal cicatrices nearly in 

 the centre of the cavity of the beaks ; cavity of the shell shallow and wide ; cavity 

 of the beaks very shallow and obtusely angular; nacre purple and very iridescent. 



So/l Paris. — Branchial uterus occupies the whole length of the outer leaf of the 

 branchia}. Branchiw large, the inner one much the larger, free nearly the Avliole 

 length of the abdominal sack. Paljn rather small, suboval, attached only at the 

 upper part of the posterior edges. Mantle thin, thickened at the margin. Branchial 

 opening large, with numerous small deep brown papillae on the inner edges. Anal 

 openiTKj rather large, with numerous very small papillae on the edges. Super-anal 

 opening rather large, and united below. Color of the mass whitish. 



Remarks. — In outline this small species is very close to similis (nobis), but it is a 



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