17 



UNIO COSTATUS. 



Plate VII. 

 DESCRIPTION. 



Shell suboval, slightly ventricose anteriorly; disk 

 with profound oblique undulations, numerous and re- 

 curved behind the umbonial slope; umbo narrow, 

 slightly prominent, oblique, nearly terminal; ligament 

 margin elevated, nearly straight; epidermis yellowish 

 brown, sometimes blackish brown, much wrinkled 

 posteriorly; within white, iridescent posteriorly, 

 waved on the margin; cardinal teeth very thick. 



SYNONYMES. 



U. costatus, Raf. Ann. gen. des Sc. Phys. vol. v. p. 49. pi. 



lxxxii. fig. 13, 14. Poulson's trans, p. 57. Say, Amer. Conch. 



No. 6. Ferus. Mag. de Zool. 

 U. undulate, Barnes. Silliman's Journ. vol. vi. p. 121, pi. 2. 



OBSERVATIONS. 



This shell very nearly approaches U. plicatus, Say, 

 and some varieties can scarcely be distinguished 

 from it. It has, in general, more undulations, a much 

 less prominent umbo, is less ventricose, and has often 

 a yellowish epidermis, which is not the case with the 

 plicatus. Not uncommon in the western waters, and 

 in the Alabama river; very large and fine in the 

 rivers of Tennessee. 



