50 



SYNONYMES. 



U. niger, Raf. Ann. gen. des Sc. Phys. vol. v. p. 25. Poul- 

 son's trans, p. 15. Say. Amer. Conch. No. 6. Ferr. Mag. de 

 Zool. 



U. crassidexs, var. b. Lam. 



U. cuneatus, Barnes. Silliman's Journ. vol. vi. p. 263. 



Cab. A. N. S. No. 2400, 2401, 2402. 



OBSERVATIONS. 



In the observations on U. congarazus, I compared 

 that species with the niger, but however the young 

 of both may resemble each other exteriorly, the car- 

 dinal teeth of the latter are always more robust and 

 less oblique, and the interior very often dark purple, 

 which is never the case with the congarceus. It dif- 

 fers most obviously from II. dilatatus, in being com- 

 paratively much shorter, and in the raised lines on 

 the posterior lunule. 



Common in the western streams, and very abun- 

 dant in the Black Warrior and Alabama rivers. In 

 Detroit river, Michigan, it has been obtained by Dr. 

 Sager. 



UNIO GIBBOSUS. 



Plate XXVII.— Fig. 1. 



DESCRIPTION. 



Shell subovate, oblique, with a submedial nodulous 

 ridge projecting at base; concentric sulci very dis- 

 tinct; posterior side with a wide furrow; umbonial 

 slope angulatcd; umbo prominent; posterior end and 



