106 



dinal and lateral teeth very thick; posterior cicatrix 

 impressed. 



OBSERVATIONS. 



I have seen but one specimen of this Unio, which 

 was found by Mr. Featherstonhaugh in Little Red 

 river, Arkansas, during his investigations of the 

 geology of that region. It has something of the 

 interior colour and general outline of U. catillus, but 

 the diameter through the umbones is far greater. 

 The beaks, if they ever occur perfect, must be ex- 

 ceedingly prominent. 



UNIO LENIS. 



Plate LVIII.— Fig. 2. 



DESCRIPTION. 



Shell obovate, rather thin, regularly convex; an- 

 terior side narrowed; extremity obtusely rounded; 

 posterior side dilated; ligament margin elevated, 

 forming with the posterior margin an arcuate line; 

 extremity obtusely rounded; base arcuate, profoundly 

 so towards the posterior extremity; umbonial slope 

 rounded; beaks slightly prominent, with rather small 

 plicae, not interrupted by well defined angles; epider- 

 mis brown-olive, wrinkled, with rather distant slender 

 green rays; within white; cardinal teeth crested, 

 oblique, single in one valve, and double in the other; 

 lateral teeth distant from the cardinal teeth. 



