UNIO DECISUS. 



Plate III.— Fig. 2. 

 DESCRIPTION. 



Shell cuneiform, oblique; anterior margin obtusely 

 rounded; posterior side produced, somewhat com- 

 pressed, with a very oblique indistinct furrow, and 

 irregular indentations, frequently obsolete; extremity 

 angular; beaks nearly terminal, eroded; umbonial 

 slope rectilinear, subangulated; epidermis finely wrin- 

 kled inferiorly, reddish-brown with obsolete brown 

 narrow rays; within white. 



SYNONYMES. 



U. decisus, Lea. Trans. Amer. Philos. Soc. new series, vol. 



iv. p. 92, pi. xii. fig. 23. 

 U. taitianus, ib. vol. v. p. 39, pi. iv. fig. 11. variety. 

 Cab. A. N. S. No. 1733. 



OBSERVATIONS. 



This species much resembles the preceding, but 

 can always be distinguished by its broader and less 

 oblique umbo, uniform absence of green rays, and 

 by its rectilinear umbonial slope, but more especially 

 by the posterior extremity, which is never so nearly 

 on a line with the base as in the former shell, which 

 with age is greatly produced and compressed, whilst 

 the present species, when old, never becomes so; 

 the cardinal teeth, also, are very different. Com- 

 mon in the Alabama river, and has hitherto been 

 found only in the state of Alabama. 



