116 OBSERVATIONS ON NAIADES, 



Unio Arc^formis. Plate XVII. fig. 44. 



Testa arc(Bformi, valde ventricosd, transversa, ; clivo posteriori latissimo et sul- 

 cum curvum habente; valvulis pracrassis; radiis capillar ibus; dentibus cardinalibus 

 crassis, lateralibus brevibus rectisque; margaritd alba. 



Shell arcseform, very ventricose, transverse ; posterior slope very wide and poss- 

 essed of a curved furrow ; valves very thick; rays hair-like ; cardinal teeth thick ; 

 lateral teeth short and straight ; nacre white. 



Hab. Tennessee river, Professor Vanuxem. 



My Cabinet. 



Cabinet of Professor Vanuxem. 



Cabinet of W. Cooper. 



Diam. 2, Length 2, Breadth 2-5 inches. 



Shell subtriangular, very ventricose, transverse, inequilateral ; pos- 

 terior slope very wide, and nearly flat except at the termination of the 

 ligament, possessed of a curved furrow enlarging from the beaks to the 

 posterior margin ; substance of the shell very thick ; beaks prominent 

 and incurved ; ligament short and thick ; epidermis yellowish brown ; 

 rays hair-like and numerous ; cardinal teeth thick and irregular ; late- 

 ral teeth short, straight and crenate ; posterior cicatrices confluent ; an- 

 terior cicatrices distinct ; dorsal cicatrices situated on the under part of 

 the cardinal tooth ; cavity of the beaks shallow and rounded ; nacre 

 white. 



Remarks. — I have been in possession of a single specimen of this 

 species for some years. It was brought by Professor Vanuxem from 

 the Tennessee river, and, being old, some of its characters have nearly 

 disappeared. I frequently examined it with great interest, and felt 

 persuaded it was a new species, although it strongly resembled the tri- 

 angularis (Barnes). A young and an adult specimen, recently sent 

 me from New York by that excellent naturalist W. Cooper, proves it 

 beyond a doubt to be a distinct species. In its rays it differs altogether 

 from the triangularis; it is dissimilar also in the thickness of the valves and 



