OF THE FAMILY OF NAIADES. 437 



posterior slope, and from the ventricosus in not being globose 

 over the umbones ; and of course is much less in diameter. 

 The quite large impression of the mantle under the callus, 

 between the lateral and cardinal teeth, is very remarkable in 

 these three species. 



9. Unio Securis. Plate XI. fig. 17. 



Testa subtriangulari, inaqitilaterali, per umbones valde complanatd ; 

 valvulis crassis ; nalibus elevatis, recurvatis, compressissimisque ; dente 

 curdi7iali magno, lalerali crasso ; ligamento breviusculo, crassoque ; 

 margaritd alba et iridescente. 



Shell sub-triangular, inequilateral, flattened over the umbones; valves 

 thick ; beaks elevated, recurved, much compressed ; cardinal tooth large ; 

 lateral tooth thick; ligament rather short and tiiick ; nacre pearly white 

 and iridescent. 



Hab. Ohio. T. G. Lea. 



My Cabinet. 

 Cabinet of T. G. Lea. 

 Cabinet of Prof. Vanuxem. 

 Cabinet of the Academy of Natural Sciences. 

 Unio depressa of Rafinesque. 

 Diam. '9, Length 1-5, Breadth 1-9 inches. 



Shell sub-triangular, transversely wrinkled, inequilateral, 

 much flattened over the umbones; substance of the shell 

 thick, often ponderous ; beaks elevated, much compressed, 

 recurved; dorsal margin angular; posterior dorsal margin ob- 

 lique; posterior margin angular; basal and posterior basal 

 margin curved ; anterior and anterior basal and dorsal mar- 

 gins round ; posterior slope flattened, this view presents the 

 shell as a long ellipsis ; epidermis olive-yellow passing into 

 olive-brown, shining and transversely wrinkled ; rays formed 

 by small spots, alternately darker and lighter than the gene- 

 ral colour of the epidermis, which cause the rays to look like 

 a minute chain, these rays are from one to two eighths of an 

 inch apart, and extend over the whole disk, the spaces be- 



