nearly 4°. The delicate colouration of the finest pearls is not due to any kind of pigment, but to the peculiar "intimate" structure of the nacre producing colour-effects through interference. Occasionally a dull pearl, when carefully peeled by mechanical means, will reveal a fine orient beneath, and be consequently greatly improved in appearance by the treatment.
Pearls are secreted by the mantle of the mollusc, and in the same way as that by which the shell itself is formed. Definite areas of the mantle have definite functions, secreting, as the case may be, either aragonite, or calcite or the horn-like substance already named. According to the position of the pearl in the region of the mantle—a position which is subject to change—so will be the nature of its successive additional coats. But it will be asked "How does the pearl, the detached pearl for example, first come into being?" Its occurrence, if not rare, is at least abnormal, and is the outcome of irritation to the mantle caused by the intrusion of some foreign body. This foreign body is usually a minute parasitic animal (a Cestode larva), but may be a grain of sand, or some other solid. The irritation stimulates the secretion of nacre, and the intruder is sooner or later covered with layer after layer. The Chinese take advantage of this response to the irritation caused by the introduction of a foreign body in the case of a fresh-water mussel (Dipsas plicata). They keep the mussels in a tank and insert between the shell and the animal rounded bits of mother-o'-pearl or little metal images of Buddha. In either case the inserted object becomes slowly coated with nacre and looks like a pearl: the little figures of Buddha generally become cemented to the shell; a specimen may be seen in the shell gallery of the Natural History Museum.
The value of pearls is increased greatly when a considerable number of well-matched specimens are got together. But the market value of pearls depends upon so many factors that, even for a single pearl of what may be called standard quality, and
