70 



CLASS CONCHIFERA. 



lined with a pearly substance, which is called mother-of- 

 pearl; this is formed by an animal deposit, and is in thin 

 layers. If, by any accident, the inner surface of the 

 shell is injured, so as to cause a fracture of the mother- 

 of-pearl, the deposit, in that place, becomes for the future 

 irregular, and a bump is gradually formed. Accidental 

 circumstances cause this bump to assume various shapes ; 

 sometimes it is oval, sometimes globular, and at others 

 pear-shaped. This kind of pearl is always originally 

 found attached to the shell by means of a small neck, or 

 footstalk, and the spot at which this neck was placed 

 can always be traced on the pearl itself. 



Mcleogriiia niarr/aril'/rra. 



But pearls are at times found loose in the shell ; — in 

 this case, the pearly matter is deposited on sou-^e ex- 

 traneous substance, such, for instance, as a grain of 

 sand, and by dissolving the pearl in an acid, this nucleus 

 can be traced. 



! Some of these round pearls are supposed to be formed 

 on a centre, consisting of the remains of a diseased 

 ovum, or egg, of the animal. 



