UNIONIDJE. 293 



terior) area running parallel with its base; again, 

 from M. macula by the anterior area of the latter slop- 

 ing upwards, and forming an angle with the fore part 

 of the shell. In M. cygneus the base slopes upwards ; 

 and M. incrassatus differs from them all by its 

 large exserted ligament, superior roughness of the 

 outside, and in having the posterior part, in a slope 

 from the umbones to the base, incrassated. (Linn. 

 Trans, xiii. 87.) 



2. ALASMODON Say. (Pearl Muscle.) 



Shell oblong-elongate ; hinge with short crested irre- 

 gular anterior teeth in the right valve, shutting be- 

 tween two in the other valve; posterior lateral teeth 

 small in the young, and wanting in the adult shells. 



Found in rapid rivers. 



This genus was established by Say : Dr. Leach 

 named it Damaris, and Dr. Turton has retained for 

 it the generic name of Unio ; but, as this genus was 

 beforejnamed by Say, and Unio is generally kept for 

 the following genus, we are induced to adopt Say's 

 name. 



123. 1. ALASMODON margaritiferus. Pearly Alasmo- 

 don. (t. 2. f. 9.) Shell ovate-elongate, rather 

 compressed, thick, heavy, black-brown, truncated 

 below; cardinal teeth thick, conical. 



Mya margaritifera. Linn. Fauna Suec. 516., Syst. 

 Nat. ii., List. Conch. 149.; Schroet. Flussc. 168. 

 t. 4. f. 1. 



Unio margaritiferus. Retz. Nov. Gen. 16.; Nil- 

 son, 103. n. 1.; Turton, Biv. 242, t. 16. f. 1., 

 Man. ed. 1. 19. f. 9. 



o 3 



