Pearl (Latin, pirula, "a pear, a jewel"), generally, as the name implies, pear-shaped, a mass of nacre, formed within the body or shell of certain molluscs - e.g. the Pearl-oyster, and the Unio, or freshwater mussel. The pearl-oyster, however, it may be remarked, is not an oyster.. The received theory of the origin of pearls is that they are caused by the presence of a foreign body within the shell or body of the animal, such body in the course of nature being covered by an extra secretion of shell-forming substance, in much the same way that a bullet or other foreign body becomes encysted within the human frame. This fact is taken advantage of to produce artificial pearls by the introduction of particles of sand or the like into the-shell of the pearl-bearing oyster. The pearl oyster is chiefly found in the Persian Gulf, Japan, the Bay of Bengal, Ceylon, and other parts of the Indian Ocean. South American waters also have produced fine pearls. The ancients were weli acquainted with the pearls of Ceylon and the Persian Gulf. The pearl-fisheries, which start in spring, employ many boats, the crews of which generally number twenty, ten of whom row, while the rest are divided into shifts of five who relieve each other in the work of diving for the oysters. The pearls, which differ according to the locality and food of the oyster, are polished with nacrepowder, are passed through sieves which separate them by size into mill-, vivadoc-, and seed-pearls. The most prized are those found in the soft parts, and are called virgin pearls. The nacre lining the shell, or mother-of-pearl, is also valuable, and is split from the shell and classed as silver - lipped, bastard - white, and bastard black. The Ceylon fisheries - which are now under Government control - have fallen off considerably, whether from overfishing, or from the migration of the oyster, for this animal has the power of separating from its beard and taking up a new position. Among notable pearls may be mentioned one possessed by Julius Caisar worth £48,000, one by Cleopatra (£80,000), one by a Shah of Persia (£180,000), and a South American pearl of.250 carats (£150,000). "Western Australia produces pearls and mother-of-pearl of considerable value. The pearl-mussel is found in most shallow rivers of Central and Western Europe.