FRESHWATER AND LAND SHELLS. 71 



Diam. 1*4, Length 1*7, Breadth 2-8 inches. 



Shell elliptical, very much inflated ; substance of the shell rather 

 thick; umbones large ; umbonial slope carinate; posterior slope with 

 three indistinct ribs on each valve ; beaks prominent ; ligament rather 

 long and somewhat thick; epidermis yellowish, with numerous dark 

 green rays over the whole disk ; cardinal teeth somewhat compressed, 

 double in both valves, enlarged at the anterior portion ; lateral teeth 

 separated from the cardinal teeth, lamellar, the inferior division of that 

 of the left valve enlarged, and longer than the superior one ; anterior 

 cicatrices distinct ; posterior cicatrices confluent; dorsal cicatrices with- 

 in the cavity of the beaks under the cardinal tooth ; cavity of the shell 

 deep and angular along the carina of the umbonial slope ; cavity of beaks 

 deep and rounded ; nacre splendid rose-colour. 



Memarks. — I owe this species, which came into my possession some 

 months since, to Professor Shepard, who procured it from his friend 

 Mr Cowper. It very closely resembles the U. ochraceus (Say), but 

 may be distinguished from it by its carinate umbonial slope, and its 

 being more inflated. The colour of all the specimens 1 have examined 

 is, in the nacre, of a rose tint. A single one has a slight salmon colour 

 mixed with it. When more are examined, there may be some found 

 entirely salmon or white. 



I owe to the kindness of Major Leconte some very fine specimens 

 from Liberty county, where they were found with the U. spinosus. 



Subgenus Margaritana. 

 M. ARCULA. Plate XXIL fig. 69. 



Testa plicatd, triangulari, valdl injiatd ; vahulis tenuissimis ; clivo umboniali carinato ; 

 natibus valde prominentibus ; epidermide alro-viride, radiatd; dentibus laminatis, irregulari- 

 bus; margarita ccerulea. 



Shell plicate, triangular, very much inflated ; valves very thin ; umbonial slope cari- 

 nate ; beaks very prominent ; epidermis dark green, radiated; teeth lamellar, irregu- 

 lar; nacre bluish. 



