MOTHER-OF-PEARL. The shells of many molluscous animals display a brilliant pearly and iridescent lustre, resulting from the peculiar manner in which the layers of calcareous matter of which they are composed have been successively formed. Such shells, even when small in size, form bright and, specially to the untutored eye, attractive ornaments, and as such are used for necklaces and similar purposes. When the shells are of sufficient size to cut and shape for purposes of utility, they become an article of some commercial importance under the name of Motherof-Pearl. This term, though applicable to all pearly shells, is in commerce principally applied to the shells of the bivalve pearl-mussel Meleagrina margaritifera, which is the principal source of the commercial product. The Meleagrina margaritifera is a native of tropical seas, and is found around the coasts of all the lands within the tropics. The shells vary in size, the largest reaching to about the dimensions of a dessert plate, with a weight of from 1 to 11 lb. They also vary in colour to a considerable extent, some being dark and smoky round the outer edge with little iridescence, others dark but possessing a rich play of colours, and the greater part pearly white with varying iridescence. The principal sources of supply are the islands of the East Indian archipelago, the Pacific islands, the north-west Australian coast, the Persian and Red Seas, and the Gulf of Panama. The largest and steadiest consumption of mother-of-pearl is in the button trade, and much is also consumed by cutlers for handles of fruit and dessert knives and forks, pocket-knives, &c. It is also used in the inlaying of Japanese and Chinese lacquers, European lacquered papier-mache work, trays, &c., and as an ornamental inlay generally. In an innumerable variety of small and fancy articles mother-of-pearl is also employed, its use being limited only by the moderate dimensions and thickness of material obtainable, and its rather brittle nature. The carving of pilgrim shells and the elaboration of crucifixes and ornamental work in mother-of-pearl is a distinctive industry of the monks and other inhabitants of Bethlehem. Among the South Sea Islands the shell is largely fashioned into fishing-hooks, a purpose for which its brilliant conspicuous appearance appears to render it suitable without the addition of any bait or other lure. Among shells other than those of Meleagrina margaritifera used as mother-of-pearl may be mentioned the Green Ear or Ormer shell (Haliotis tubercu lata) and several other species of Haliotis, besides various species of Turbo.

The pearl-shell fishery is an important industry on the north and north-west coasts of Australia, producing about 800 tons yearly, valued at over £100,000, the Papuan islanders of Torres Straits being employed as divers under European supervision, with skilled appliances. The shell of the golden-tipped variety of Avicula found here is much more valuable than the dark-edged one of the South Seas. The value of the fisheries depends much more on the shell than on the occasional pearls found, which indeed are sometimes, along with the "fish," a perquisite of the diver ; but on the west coast, about Shark's Bay, a smaller variety of the same mollusc produces valuable pearls, their exciting cause being possibly present there in greater abundance. That the pearl itself is not due to disease, or to the presence of any irritating cause, seems clear from the fact that the mollusc can reject it at will, and often does so when taken (for which reason the diver, in seizing him, at once places his hand over the opening so as to close the shell) ; but it is believed now that the pearl is secreted and held ready to be dissolved by the powerful acid of the sac, and spread in nacreous layers over the spot irritated by the borer (Pholax sp.). Accordingly pearls are seldom found in the young "fish," whose shells are much harder outside, and not susceptible to such attacks. A mass of nacreous layers formed round a point of irritation or " blister " can sometimes be cut out of the shell, and might easily be mistaken for (and sold as) a pearl, but it is never quite perfect all round and is always hollow. Sometimes, after having secured the loose pearls, the fishermen deposit the mollusc again, unharmed, in a secure and accessible locality, and repeat the process for three and four years successively.