26 DESCRIPTION OF NEW 
~ 
growth; posterior slope compressed and raised into a carina and very dark-green; um- 
bonial slope but slightly raised and rounded ; cardinal teeth very small, compressed, very 
oblique, double in both valves; lateral teeth long, lamellar, somewhat curved and abrupt 
at the posterior end, separated from the cardinal tooth; anterior cicatrices distinct; pos- 
terior cicatrices confluent; dorsal cicatrices placed in the cavity of the beaks and on the 
under side of the plate; palleal cicatrices very indistinct ; cavity of the shell rather shal- 
low and rounded; cavity of the beaks shallow and subangular; nacre bluish and beauti- 
fully iridescent. 
Remarks.—A single specimen only was received by Mr. Wheatley from Georgia. In 
outline it is very much the same as J. radiatus, but it is a much thinner shell, and in its 
polish, rays, and thinness allied to U. Modioliformis, (Nobis.) ‘The rays in this specimen 
are so numerous, broad and dark, as to give the shell quite a tenebrose appearance, almost 
obliterating the yellowishness of the epidermis. 
I name this species after Professor Prevost of Geneva, who, in his able memoir, “ De la 
generation chez les moules des peintres” (Unio pictorum) was the first by anatomical de- 
monstration to prove the Natades to be dicecious; it having been previously supposed the 
male and female organization existed in the same individual. (See Mem. de la Soc. de 
Hist. Nat. de Geneve, March 17, 1825.) 
Unio Powettn. PI. XIX. Fig. 25. 
Testa levi, ellipticd, subinflata, inequilaterali, postice angulatd; valvulis subtenuibus 3 natibus subpromi- 
nentibus ; epidermide olivacea, nitidd, eradiatd ; dentibus cardinalibus parvis, subcompressis, obliquis, acumi- 
natis; lateralibus prelongis subcurvisque; margarita albé et iridescente. 
Shell smooth, elliptical, somewhat inflated, inequilateral, angular behind; valves rather thin; beaks rather 
prominent; epidermis olivaceous, shining, without rays; cardinal teeth small, rather compressed, oblique, acumi- 
nate; lateral teeth very long and somewhat curved; nacre white and iridescent. 
Hab. Saline river, Arkansas, J. Hale, M. D:, and Professor Powell. 
My cabinet and cabinets of Dr. Hale and Prof. Powell. 
Diam. 1.1, Length 1.8, Breadth 3.1 inches. 
Shell smooth, elliptical, somewhat inflated, inequilateral, angular behind; substance of 
the shell rather thin, thicker before; beaks rather prominent and placed towards the 
anterior margin; ligament long and rather thick; epidermis olivaccous, shining, without 
rays, with distant, indistinct marks of growth; posterior slope rather depressed, with two 
slightly impressed lines from the beaks to the posterior margin; umbonial slope sub- 
angular; cardinal teeth small, rather compressed, oblique, pointed, crenulate, single in the 
right and double in the left valve; lateral teeth very long, somewhat lamellar, curved, 
separated from the cardinal tooth; anterior cicatrices distinct; posterior cicatrices con- 
fluent; dorsal cicatrices placed on the under side of the plate between the teeth; palleal 
cicatrices impressed; cavity of the shell rather deep; cavity of beaks deep and obtusely 
angular; nacre white and iridescent. 
Remarks.—I owe to the kindness of Dr. Hale and Prof. Powell several specimens of 
this species of both sexes. It belongs to that group of which cariosus, Say, may be con- 
