Pearl is, in fact, a calcareous secretion by the fish of bivalve shells; and principally by such as inhabit shells of foliated structure, as sea and fresh water muscles, oysters, &c. A pearl consists of carbonate of lime, in the form of nacre, and animal matter arranged in concentric layers around a nucleus; the solution indicating no trace of any phosphate of lime. To this lamellar structure the irridescence is to be ascribed. Each layer is _presumed_ to be annual; so that a pearl must be of slow growth, and those of large size can only be found in full-grown oysters. The finest and largest are produced from the Meleagrina margaratifera, (_Lamarck_,) a native of the sea, and of various coasts. A considerable number are likewise taken from the Unio margaratifera, which inhabits the rivers of Europe; and, it is singular, as remarked by Humboldt, that though several species of this genus abound in the rivers of South America, no pearls are ever found in them. The pearls are situated in the body of the oyster, or they lie loose between it and the shell; or, lastly, they are fixed to the latter by a kind of neck; and it is said they do not appear until the animal has reached its fourth year. Naturalists have much disputed the formation of pearls. Mr. Gray justly observes they are merely the internal nacred coat of the shell, which has been forced, by some extraneous cause, to assume a spherical form. Lister, on the other hand, states "a distemper in the creature produces them," and compares them with calculi in the kidneys of man. But, as observed by a more recent inquirer,[12] "though they are accidental formations, and, of course, not always to be found in the shellfish which are known usually to contain them, still they are the products of a regular secretion, applied, however, in an unusual way, either to avert harm or allay irritation. That, in many instances they are formed by the oyster, to protect itself against aggression, is evident; for, with a plug of this nacred and solid material it shuts out worms and other intruders which have perforated the softer shell, and are intent on making prey of the hapless inmate: and it was apparently the knowledge of this fact that suggested to Linnaeus his method of producing pearls at pleasure, by puncturing the shell with a pointed wire. But this explanation accounts only for the origin of such pearls as are attached to the shell; while the best and greatest number, and, indeed, the only ones which can be strung, have no such attachment, and are formed in the body of the animal itself. 'The small and middling pearls,' says Sir Alexander Johnston, 'are formed in the thickest part of the flesh of the oyster, near the union of the two shells; the large pearls almost loose in that part called the beard.' Now, these may be the effect merely of an excess in the supply of calcareous matter, of which the oyster wishes to get rid; or, they may be formed by an effusion of pearl, to cover some irritating and