Éléments de conchyliologie conchyliologie élaboré les éléments d'/ préparé pour l'utilisation d'écoles et collèges elementsofconcho Année : 1844 wild west00MELEA 76 A. GRIN-perles. Fig. 96. MELEAGR5NA MARGARITIFERA. été divisé, peut-être sans sutlicient raison, en deux genres, et AvicuLA Meleagrina, selon que la coquille est avec ou sans l'aile-comme prolon- obligations, et la charnière est armé d'une dent ou non provisionnée avec une protubérance semblable : la coquille de ce dernier est presque equivalve, et le passage du byssus produit dans chaque vanne d'un cran. Le Meleagrince sont plus scaly ex interne de

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Elements of conchology Prepared Elements of conchology / Prepared for the use of schools and colleges elementsofconcho00rusc Year: 1844 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

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