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. Dith. j). 242, f. IG, /. 1. 

 Unio margaritifera. Drap. 132. t. 11. f. 5. 

 Mya margaritifera. Limi. 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. 



