26 DESCRIPTION OF NEW 



growth ; posterior slope compressed and raised into a carina and very dark-green ; um- 

 honial slope bnt sliglitly 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 cicati ices 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- 

 full}' iridescent. 



Remarks. — A single specimen only was received by Mr. Wheatley from Georgia. In 

 outline it is very much the same as U. radiah/s, 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 ycllowishness of the epidermis. 



I name this species after Professor Prevost of Geneva, who, in his able memoir, "De la 

 generation chez les monies des peintres" (Unio piclorum) was the first by anatomical de- 

 monstration to prove the Naiades to be dioecious; it having been previously supposed the 

 male and female organization existed in the same individual. (See Mem. de la Soc. dc 

 I'Hist. Nat. de Geneve, March 17, 1825.) 



Unio PowELLii. PI. XIX. Fig. 25. 



Testa Ixvi, elliplicd, subinjlata, insequilaterali, postice angtilald; valvuUs suhtemiibuH ; nutibus sidr/jromi- 

 nentlbus; cpidcrmide oHvaccn,}}ilidd, crad'mta ; dcntibvs cardvudibus parvis, sitbcompressis, obliqids, (tciinii- 

 nalis; lateralibus prselongis siibcurvisque ; margaritd albd ct iridcscenle. 



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 olivaceous, 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 se.ves. It belongs to that group of which cariosus, Say, may be con- 



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