36 



extremity truncated, direct; epidermis with green 

 rays, obsolete anteriorly, distinct and very unequal 

 posteriorly; umbo slightly prominent; beaks with an- 

 gular furrows, pointed; within white, iridescent; car- 

 dinal teeth compressed, oblique, that of the left valve 

 double or trifid; lateral teeth of the left valve imper- 

 fectly divided. 



SYNONYME. 



U. viridis, Raf. An. gen. des Sc. Phys. vol. v. p. 27. Poulson's 



trans, p. 19. 

 Cab. A. N. S. No. 2105. 



OBSERVATIONS. 



The first specimen of this species which came 

 under my observation, was found in the Schuylkill 

 river, near Philadelphia, by Mr. Hyde, who kindly 

 offered me the use of it, as I supposed it to be unde- 

 scribed. Comparing it, however, with a single valve 

 from the Kentucky river, in Mr. Poulson's cabinet, 

 it proved to be of the same species with that speci- 

 men which was labelled viridis by Rafinesque. Mr. 

 Hyde has received specimens from a small stream 

 near Lancaster, and from the Juniata river. One 

 from the latter locality has been selected for the figure 

 and description. It is allied to U. heterodon, but can- 

 not be confounded with that species. 



