110 OBSERVATIONS ON NAIADES, 



from the cardinal teeth and pointing towards the basal margin ; ante- 

 rior cicatrices distinct, the great one forming a deep pit ; posterior 

 cicatrices distinct, the smaller one being placed at the end of the late- 

 ral tooth ; dorsal cicatrices situated on the under part of the cardinal 

 tooth ; cavity of the beaks deep and angulated : nacre beautifully flesh 

 coloured, very rarely white. 



Remarks. — This very beautiful and interesting shell has heretofore 

 been considered as the U. mytiloides (Rafin.). It does not however 

 answer either to the description or figure of that author. It may be 

 easily distinguished from any described species of this genus by its ex- 

 ceedingly elevated beaks, and beautiful nacre. In young individuals, 

 indistinct rays may be observed on the beaks. 



Unio Trigonus. Plate XVI. fig, 40. 



Testa subtriangulari, inflatd, prcBclivo umboniali{quod carinatum est), depressd; 

 valvulis crassis, natibus prominentibus, incurvis; dentibus cardinalibus magnis, 

 lateralibus magnis et subcurvis; margaritd alba et iridescente. 



Shell subtriangular, inflated, depressed before the umbonial slope whicii is cari- 

 nate ; valves thick ; beaks prominent, incurved ; cardinal teeth large; lateral teeth 

 large and slightly curved ; nacre pearly white and iridescent. 



„ , C Ohio river at Cincinnati, T. G. Lea. 

 c Ohio river at Louisville, T. H. Taylor. 

 My Cabinet. 

 Cabinet of Professor Vanuxem. 

 Cabinet of J. Ronaldson. 

 Cabinet of the Academy of Natural Sciences. 

 Diam. 1'5, Length 2, Breadth 2-3 inches. 



Shell subtriangular, inflated, nearly equilateral, depressed before the 

 umbonial slope, angular behind ; umbonial slope carinate ; basal mar- 

 gin emarginate; substance of the shell thick, beaks prominent, in- 



