the other, with a remote indistinct tooth on Ccich side ; cha- 

 racters that indicate an approach to Cyrena. Some of 

 these genera are intimately linked together by intermedi- 

 ate species, particularly the Unio and Anodonta, there be- 

 ing one or more in our streams, that it is somewhat diffi- 

 cult to determine which of the two genera it ought to be 

 placed in. Consequently those naturalists, who advocate 

 the union of such genera as are connected by intermedi- 

 ate species, will be compelled to unite these, and on the 

 same principle, they must reject great numbers of genera 

 long since firmly established. 



4 



UNIO TRIANGULARIS. 



SPECIFIC CHARACTER. 



Shell triangular, very convex ; anterior margin forming 

 a right angle with ihe disk. 



SYNONYMS. 



Unio triangularis. Barnes. Silliman\s Journal, vol. 



vi. p. 272, pi. 13, fig. 17. 

 Unio c^uneatus. Swttmson. TillocKs Magazine, De 



cember, 1823. 



DESCRIPTION. 



Shell triangular, very convex, gaping at the extremities : 

 anlerior margin very much llattened, forming a right an- 

 gle with the disks ; having longitudinal slightly elevated 



PLATE IV. 



