68 

 UNIO NEXUS. 



Plate XXXVIIL— Fig. 1. 



DESCRIPTION. 



Shell triangular, sub-rhomboidal, much inflated, 

 thick; umbo prominent; posterior slope much de- 

 pressed, with a broad, shallow groove, which extends 

 from the beak to the posterior margin; posterior 

 margin forming nearly a right angle with the base, 

 obtusely emarginate in the middle by the termination 

 of the groove; umbonial slope carinated, in conse- 

 quence of the depression of the posterior side; lunule 

 very short; epidermis yellowish-brown, obsoletely ra- 

 diated; within white; cardinal teeth direct; lateral 

 teeth short, with but little obliquity. 



SYNONYMES. 



U. nexus, Say. Trans. Journ., vol. iv. p. 527, 1831. Amer. 



Conch., pi. li. 

 U. arcteformis, Lea. Trans. Amer. Phil. Soc, new series, vol. 



iv. p. 116, pi. xvii. fig. 44. 

 Cab. A.N. S.No. 20410. 



OBSERVATIONS. 



Mr. Say remarks that this species is "related to 

 tric/ueter, Raf., but differs in the greater prominence 

 of the superior portion of the anterior [posterior] 

 margin." There is also a great difference in the 

 teeth and in the markings of the epidermis, which in 

 the nexus are filiform rays, but in the triqueter gene- 

 rally sagittate spots. Not uncommon in the Cumber- 



