UNITED STATES AND NORTHERN MEXICO. 23 



Shell smooth, elliptical, somewhat inflated, slightly flattened at the sides, very 

 inequilateral, obtusely angular behind, rounded before ; substance of the shell rather 

 thin, thicker before; beaks slightly prominent; ligament short, thin and brown; 

 epidermis yellowish olive, shining, with rather indistinct rays disposed to be spotted 

 and somewhat distant lines of growth ; umbonial slope slightly raised and obtusely 

 angular ; posterior slope rather low, slightly grooved and slightly emarginate at posterior 

 margin ; cardinal teeth small, compressed-conical, crenulate, single in the right and 

 double in the left valve ; lateral teeth long, lamellar and slighth- curved ; anterior 

 cicatrices distinct and well impressed; posterior cicatrices confluent and slightly im- 

 pressed ; dorsal cicatrices placed under the plate behind the cardinal tooth ; cavity 

 of the shell rather deep and wide ; cavity of the l:ieaks shallow and subangular ; nacre 

 white or purplish and very iridescent. 



Remarls. — Two specimens only were received from Prof. Tuomey. The smaller 

 one imperfect and more yellow in the epidermis. The larger one, figured, is purple 

 tinted in the cavity and in the teeth, the other is white with yellow sjwts made by 

 deposit of epidermal matter in the layers of the nacre. It is near to (//«&e?- (nobis), 

 inclining towards lymilhts (nobis). It is easily distinguished from the former by 

 being thicker, more oblique, less rayed, being more inflated and of a darker color in 

 the epidermis. It is not so dark, so oblique nor so transverse as xiusillus. Near 

 the anterior margin both specimens are slightly sulcate. The beaks were not suffi- 

 ciently perfect to shew any undulations. 



Unio virescexs. pi. 55, fig. 1G6. 



Testa lajvi, elliptici, subinflata, postice angulata, valdO inaequilaterali ; valvulis .«ubtenuibus, antic6 crassiori- 



bus ; natibus prominulis ; epidermide nitida, luteo-viridi, obsoletiV radiata, postici' tcncbrosa ; der.tibus 



eardinalibus parvis, acuiuinatis, crenulatis, in utroque valvulo duplicibus; lateralibus longis, laraellatis 



rectisque ; margarita alba et valdc; iridesceute. 



Shell smooth, elliptical, subiuflated, angular behind, very inequilateral; valves 



rather thin, thicker before ; beaks a little prominent ; epidermis shining, yellowish 



green, obsoletely radiate, dark behind ; cardinal teeth small, acuminate, crenulate and 



double in both valves; lateral teeth long, lamellar and straight; nacre white and 



very iridescent. 



Proc. Acad. Nat. Sci. 1858, p. 40. 

 Ilah. — Tennessee River at Tuscurabia, Alabama. B. Pybas. 

 My cabinet and cabinets of Mr. Pybas and Mr. Thornton. 

 Diam. -9, Length 1-3, Breadth 2-2 inches. 



Shell smooth, elliptical, somewhat inflated, very inequilateral, angular behind and 

 rounded before ; substance of the shell rather thin, thicker before ; beaks a little promi- 

 nent; ligament short, rather thick and dark brown; epidermis shining, yellowish 



