Though one may count his pearls by the score, the hoard may be valueless. Upon such examples entertaining, if not valuable, experiments may be made without affectation or giving hostages to fortune. In all the little deformed specimens thus dissected the core has been found to consist of a foreign substance, generally what seemed under a microscope of limited power a speck of dirt. The heart of one was a blob of mud, which gave off a most baleful vapour. This was the result of the house-cleaning of a common, edible rock oyster, and the pearl, dirty green and lustreless, merely a thin casket, for the noisome mud had not solidified. The care with which the impurity had been rendered innocuous demonstrated the correct ideas of the oyster on sanitation. No doubt the germ of the special form of tape-worm which troubles oysters, irritates to pearl-making, and passes through other transformations in other hosts, and completes its cycle in the body of a shark, would be too minute for inexpert detection. The fact that molluscs do intern foreign and obnoxious substances is testimony to their decency and love of cleanliness, and so may the pearl be still accepted as the embodiment of purity. Though all its little soul be dirt, the pearl is pure, and but for the dirt or the germ of a filthy ailment it would not be pearl. So many molluscs produce pearls that it would be absurd for the great oyster family to set up exclusive rights. They do not, for your oyster is ever humble even when tenanted with a rivalless pearl. On the coast of North Queensland, within the Great Barrier Reef, pinnas of at least two species are among the producing agents, which, covering a wide range, seem to meet in two distinct genera, far apart in appearance and habit. There is the frail, flat, translucent “window-shell” (Placuna), the valves of which fit so closely that the poor little inhabitant is squeezed to a wafer, a film, a fragment of muscle. Yet in some localities nearly every individual has a pearl, pretty in tint, but too minute to be of value. An allied species is common on the coast of China, where the pearls are collected for export to India, to be reduced to lime by calcination for the use of luxurious betel-nut chewers. These almost microscopic pearls are also burnt in the mouths of the dead who have been influential and wealthy. Coal-black pearls occur in one of the pinnas, the interior of which is sooty, shot with iridescent purple, and since the pearl, whether produced by oyster, mussel, pinna, or window-shell, is generally more brilliant than the containing shell, that of the black pinna, with the high lights of its environment concentrated, may be a gem of surpassing novelty and beauty. But the habitual product of this pinna is small, dull, mud-tinted or brown, and of no value whatever. Another of the genera grows “seed” of excellent lustre, corresponding with the azure brightness of the shell.