gg NEW UNI0N1D/E OF THE UNITED STATES. 



tenebro-rufo-fusca, obsolete radiata, transverse striata; dentibus cardinalibus minimis, subcomprcssis, 

 in utroque valvulo duplicibus ; lateralibus praslongis, arcuatis ; margarita purpurescente et valde 

 iridescente. 



Shell smooth, arcuate, much compressed, flattened at the sides, inequilateral triangu- 

 lar behind, round before; valves rather thin, slightly thicker before and behind; 

 beaks slightly prominent ; epidermis dark reddish brown, obscurely rayed, trans- 

 versely striate ; cardinal teeth very small, rather compressed, double in both valves ; 

 lateral teeth very long, arcuate ; nacre purplish and very iridescent. 

 Proc. Acad. Nat. Sei. 1861, p. 40. 

 Hah. — Alabama. E. R. Showalter, M. D. 

 My cabinet and cabinets of Dr. Showalter and Dr. Hartman. 

 Diam. -7, Length L7, Breadth 3-3 inches. 



Shell smooth, arcuate, very much compressed, very much flattened at the sides, in- 

 equilateral, biangular behind and rounded before; substance of the shell rather thin, 

 slightly thicker behind and before ; beaks slightly prominent ; ligament long, thin 

 and concealed; epidermis dark reddish brown, obscurely rayed, with very distant 

 marks of growth; umbonial slope very low and obtusely angular; posterior slope 

 very narrow, raised into a high carina, almost constituting a wing ; cardinal teeth 

 very small, rather compressed and double in both valves ; lateral teeth very long, 

 arcuate and thickened at the posterior extremities; anterior cicatrices distinct, 

 rather small and slightly impressed ;' posterior cicatrices confluent and very slightly 

 impressed ; dorsal cicatrices placed in a curve line from the centre of the cavity of 

 the beaks along the under side of the arch of the lateral teeth ; cavity of the shell 

 very shallow and very wide ; cavity of the beaks very shallow, scarcely perceptible ; 

 nacre purplish and very iridescent. 



Remarhs. — A single specimen only of this remarkable species is before me. It re- 

 minds one at once of percoarctatus (nobis). Like it, it is exceedingly compressed, has 

 the broad, flat posterior slope, with its biangular, obtuse termination, and has its 

 loose, closely striate and dark epidermis. It differs in the outline in having an arcuate, 

 basal line, in having arcuate lateral teeth, and in the cardinal teeth being nearly 

 direct, while in percoarctatus, the cardinal teeth are larger and nearly perpendicular to 

 the basal margin of the shell. The longest perpendicular line is about one-third from the 

 posterior margin. The long, lateral teeth are very thin and arched about the middle, 

 and very much thickened at the posterior termination. It is imperfectly double 

 in the left valve only. It is greatly to be regretted that a single individual only is 

 under examination, as others might vary in some of the remarkable characters of this 

 one. The thinning of the central parts of the substance of the valves may differ 

 in other individuals. 



