55 



U. fabalis, Lea. Trans. Amer. Philos. Soc. vol. iv. p. 96, pi. x. 



fig. 16. 

 Cab. A. N. S. No. 2404. 



OBSERVATIONS. 



Mr. Say observes that "the robust teeth and the 

 thickness of the whole shell have induced many, and 

 myself amongst the number, to consider this shell as 

 the young of gibbosus, Barnes, [dilatatus, Raf.] but a 

 very slight examination serves to show that it is very 

 distinct. The young of that species is always much 

 more elongated transversely, not so thick, with the 

 beaks much undulated," &c. 



It is usually smaller than the specimen represented, 

 and, except the acidissimus, is the least of North 

 American Uniones yet described. Dr. Sager found 

 it in Detroit river, Michigan, and it is not uncommon 

 in the Ohio and its tributaries. The figure repre- 

 sents a fine specimen, of more than ordinary size, 

 which we owe to the politeness of Professor J. Green. 



UxNIO FRAGILIS. 



Plate XXX. 



DESCRIPTION. 



Shell ovate, thin, fragile, somewhat inflated over 

 the umbonial slope; disks flattened towards the base; 

 hinge margin elevated into a large wing, connate; 

 within bluish and highly iridescent; cardinal teeth 

 very oblique, compressed, disposed to be single in 

 each valve; lateral teeth arcuate. 



