Book V. 
MOLLUSCOUS ANIMALS. 
§ I. Bivalves, or those having two shells. 
THE PEABL OYSTER. (Avicula Margaritifera.) 
Who that sees the beauty and delicacy of pearls would 
imagine that they were the production of disease ? 
Such, however, is the case, as they are either formed in 
the body of the oyster which inhabits the shell ; or they 
rise from cracks in the shell itself, the delicate, silvery, 
half-transparent lining of which forms the substance 
generally called Mother-of -Pearl, or Nacre. Their for- 
mation is generally caused by the introduction of some 
foreign body between the mantle or skin of the animal 
and its shell ; the irritation thus produced causes succes- 
sive coats of pearly matter to be deposited on the in- 
truding object, and thus the pearl is formed. The best 
pearls are those which are fairly imbedded in the sub- 
stance of the mantle. These shells are found in the 
