FHESH WATER AND LAND SHELLS. 269 



Hab. Alexandria, Louisiana, J. Halo, M. D. 



My cabinet and cabinets of Dr. Hale, and Acad, of Nat. Science. 

 Diam. .7, Length 1.3, Breadth 2.2 inches. 



Shell smooth, elliptical, rather convex, inequilateral, rounded behind; substance of the 

 shell rather thin; beaks small, slightly prominent, undulated at the tip; ligament rather 

 long and thin; epidermis shining, black, striate, in the young radiate all over the disk; 

 marks of growth rather distant; posterior slope compressed into a small carina; uinbonial 

 slope slightly raised and rounded; cardinal teeth rather large, somewhat compressed, 

 oblique, acuminate, crenulate and double in both valves; lateral teeth long, lamellar, some- 

 what curved and separated from the cardinal tooth; anterior cicatrices distinct; posterior 

 cicatrices confluent; dorsal cicatrices placed near the edge of the plate and on the car- 

 dinal tooth; cavity of the shell rather shallow; cavity of the beaks shallow and subrotund; 

 nacre white and very iridescent. 



Remarks. — This species is nearly allied to U. Haleianus, (Nobis.) but dillers in the 

 outline, and is not so wide; in the epidermis, which is blacker and more shining, and in 

 the beaks, the undulations of which are more numerous and less regular. It belongs to 

 that group of which U. Xa.shvillianus, (Nobis,) may be considered the type, the beaks being 

 very much, in their undulations, of the character of that species. All the adults, male 

 and female, of which I have many specimens, are very black and apparently without rays, 

 but when held up to a strong light, delicate obscure rays may be observed, particularly 

 about the umbonial slope. In the individuals of one-third growth and less, the epidermis 

 is dark-green and covered with rays. The female dillers very much in outline from the 

 male, and is much inflated on that portion of the disk which tends to the posterior basa] 

 margin. In this very much enlarged female characteristic, this species resembles Nash- 

 villiamu, Haleianus and parvus. This enlargement makes that margin obliquely truncate. 



I'.vio Prevostianus. PI. XIX. Fig. 21. 



/ mi, eOipticd, nibcompressd, valde inequilaterali, poatice rolundatd; valvulis pertenuibus; natibus 



parvu, prominulia, ad lipids undidatis; epidermide luted, tenebroso-radiatd, politd; dentibus cardinalibus 

 parviarimiu, compressis, perobliquis; lateralibus longit, lamdlatis subcurvisque ; margarila cseruled el 

 iriih tcenle. 



Shell smooth, elliptical, rather compressed, very inequilateral, rounded behind; valves very thin ; beaks small, 

 slightly prominent, undulated at the lip; epidermis yellow, very much and darkly radiated, polished; cardinal 

 teeth very small, compressed, very oblique; lateral teeth long, lamellar and somen hat curved; nacre bluish and 

 iridescent. 



Hab. Eutowah River, Georgia, Mr. C. M. Wheatley. 

 Cabinet of Mr. Wheatley. 

 Diam. .7, Length 1.2, Breadth 2.3 inchi 



Shell smooth, elliptical, rather compressed, very inequilateral, rounded behind; substance 

 ol tin' .-bell very thin; beaks small, slightly prominent, undulated ;it the tip; ligament! 

 and thin; epidermis polished, yellow, 'with numerous broad dark-green rays covering 

 nearly the whole disk, lighter before and very dark behind, with very distant marks ol 

 vol. x. — 51 



