152 BIVALVES. 



become a thick, tough, coriaceous* skin for its 

 protection. The animal is capable of extending 

 this tube to the distance of nine or ten inches, 

 and of contracting it to about three, but cannot 

 withdraw it entirely into the shell. 



MYA Margaratifera. 



PEARL-BEARING OYSTER. 



Specific Character. Shell strong, ponderous, 

 thick ; shape ovate, oblong, front compressed, 

 margin a little contracted in the middle, giving 

 a somewhat curved outline to the circumference ; 

 hinge consisting of a cardinal tooth in one valve, 

 which is thick, obtusely conical, and locking into 

 a bifurcatedf tooth in the other valve ; shell 

 about two inches long, and four broad ; covered 

 with a black epidermis decorticated J at the um- 

 bones ; inside a greenish pearly hue. 



This shell is one of a very interesting group 

 now formed into a distinct genus called Unio. It 

 inhabits only rocky torrents, and the precipitous 

 streams of mountainous districts. Many are found 

 in the cataracts and rapid rivers of America, and 

 their solid and thick shells manifest the providen- 



* Coriaceous, like leather, derived from the Latin cori m, 

 akin, leather. 



t Bifurcated, having a fork of two teeth : from the Latin 

 bis, twice ; and furca, a fork. 



J Decorticated, having the bark or outer skin taken off ; 

 from the Latin de, off : and cortex, the bark of a tree. 



