152 BIVALVES. 



become a thick, tough, coriaceous* skin for 

 its protection. The animal is capable of ex- 

 tending 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 bifurcated f tooth in the other valve ; shell 

 about two inches long, and four broad ; covered 

 with a black epidermis decor ticatedj at the urn- 

 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 dis- 

 tricts. Many are found in the cataracts and 



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

 skin, leather. 



f 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. 



