19 



III. UNIO. 



Shell bivalve, transverse, equivalve, inequilateral, a 

 little open at the sides ; with the beaks decorti- 

 cated and eroded. Hinge with an obtuse conic 

 jagged primary tooth, which in one of the valves 

 is divided into two lobes ; and a remote cardinal 

 depression on the anterior side ; lateral teeth none. 

 Ligament external. 



9. UNIO margaritiferus. Shell oblong and slightly 

 curved, angular at the anterior end. 



Unio margaritiferus. Turt. DM. p. 242. 1. 16. f. 1. 

 Unio margaritifera. Drap. 132. t. II. f. 5. 

 Mya margaritifera. Linn. Gmel. Mont. 



Shell thick, strong, two inches or more long, and four 

 or five broad, covered with a black wrinkled skin, 

 which fringes the front margin ; inside blackish- 

 white mixed with green, with a blackish border 

 round the edge : the front margin is a little con- 

 tracted in the middle, giving a somewhat curved 

 outline to the figure. 



Young shells have the interior marginal edge, under 

 the ligament, raised into a kind of laminar process ; 

 but the teeth distinguish it from the Anodon, 

 and the terminal cavity from the Mysca. 



Found in most rapid rivers. 



