The known facts about a pearl are these. It is composed of about ninety-two per cent. carbonate of lime, about six per cent. organic matter and a little over two per cent. water in combination almost identical with the lining of the shell in which it grows and similar to the mineral aragonite. In construction it is usually a series of layers, which can sometimes be peeled off entirely, each one successively enveloping its predecessors apparently as an independent structure though itself composed of a number of thin lapping waves. Upon cutting through these layers the divisions appear as a series of rings and the intervals, though composed of many thin waves, appear compact. It grows spherically or with such modifications as the exigencies of position in the shell would reasonably account for. These facts seem to justify the hypothesis that a foreign substance upon entering the shell of a pearl oyster is at once enveloped or washed in the creature's exudations; that the organic matter of the secretions forms a filmy envelope in which the mineral contained in them is precipitated or crystallizes in wave-like layers of crystals of great tenuity, and that as these layers harden the process is repeated, and that during the process the creature either revolves the object, or about it, as it is free, or fastened to the shell. It is also possible that changes in the organic matter interwoven with the calcium carbonate may produce some chemical action resulting in the crystallization of the lime, and the crystallization in turn be provocative of another deposit, each process in turn being almost simultaneous and that the process is continued until a paucity of mineral in the exudations induces a rest for recuperation, after which the process is repeated, the result being a succession of composite skins as we find them. Whatever the cause, it is evident in all parts of the shell and in the pearl that continuity of construction is periodically arrested to be resumed upon exactly the same plan, except that the material used in the succeeding layer of the pearl may be formed occasionally like another of the shell sections though usually it is like the preceding one. Marked differences in the same skin occur more frequently in the pearl formations of univalves. The skins of the abalone pearl especially, are frequently nacreous in part only. Pearl oysters are found in immense numbers on banks having a calcareous foundation. They are extraordinarily prolific, the spat of one oyster being estimated at upwards of several hundred thousands to millions, so that were it not for the natural enemies of their young and the liability of being swept away and scattered by storms before they have anchored, the banks would be over-crowded with the myriads produced. Some idea of the numbers may be gained from the fact that during the fishing season the Ceylon divers raise about one million each day.