76 



MELEA GRIN A.— PEARLS. 



Fig. 96. MELEAGR5NA 



MARGARITIFERA. 



been divided, perhaps without sutlicient reason, into two genera, 

 AvicuLA and Meleagrina, according as the shell is with 



or without the wing-like prolon- 

 gations, and the hinge is armed 

 with a tooth or unprovided with a 

 similar protuberance : the shell of 

 the latter is nearly equivalve, and 

 the passage of the byssus produces 

 in each valve a notch. The 

 Meleagrince are more scaly ex- 

 ternally than the Aviculge. Their 

 nacre is sometimes very thick and 

 very brilliant ; and the extravasa- 

 tion of the liquid destined for the 

 periodical augmentation of the in- 

 terior of the shell, frequently gives 

 rise to isolated deposits of this 

 beautiful nacre, forming pearls. 

 The shell of the Pearl Oyster is 

 nearly semicircular, scaly and greenish brown externally; it 

 grows to considerable size, and is to be found on the coast of 

 Ceylon, in the Persian Gulf, the Gulf of Mexico, and in many- 

 other localities, where it occurs in extensive beds, attached by 

 its byssus to submarine rocks : it is the object of an active 

 fishery. 



16. Pearls, as stated above, are bodies of the same nature 

 as the brilliant nacre lining the shell : they are composed of 

 concentric layers of nacre very closely applied one over the 

 other, like the coats of an onion, and are produced whenever 

 this matter, instead of being spread out in thin layers over those 

 already deposited, constitutes small isolated masses like little 

 drops, or adhering to the shell by a mere pedicle. Their for- 

 mation depends upon a kind of disease, or, at least, upon an 

 anomalous activity of the secretory process which gives rise to 

 nacre : hence every circumstance that stimulates this secretion, 

 such as the presence of a grain of sand or other foreign body 

 betwixt the shell and the mantle of the animal, lends to bring 

 about this formation. Pearl Oysters are not the only mollusks 

 that produce pearls. All shells that are internally nacreous 

 may contain them. Patellae, Haliotides, and our common mus- 

 sels sometimes contain them, and it is not uncommon to find 

 them in a sort of large mussel (Unio) which inhabits the great 

 rivers of northern Europe and the United States ; but the 



16. What are pearls? 



