The shells of mollusks are applied to a surprising variety of purposes, from paving roads to making shirt-studs, while their natural beauty has suggested their utilization as ornaments in a hundred ways. We cut them up by the million into buttons and various small objects, such as parasol handles, and polish and fashion them into all sorts of knickknacks, thus giving employment to thousands of persons. Many ship-loads of shells are brought to New York from the West Indies every year for such purposes. I need not dwell upon this, but turn to the interesting subject of pearls. [Illustration: CASSIDIDÆ. Helmet-shell (_Cassis flammea_).] Mother-of-pearl is the bright inside surface, or _nacre_, of the large oyster that gives us pearls, which are themselves composed of the same substance formed in a nodule around some intruding substance, like a grain of sand, which irritates the mollusk’s skin until it is made smooth and comfortable by this iridescent coating. Bivalves yielding this beautiful substance exist in various parts of the world; but in America the only fishery for the pearl-oyster is in the Gulf of California, and that is by no means as productive as it used to be. The season for pearl-fishing on the Pacific coast of Mexico is from June to December, but the diving can be done only in good weather, and for about three hours at the time of low water, since the tide there rises twenty feet, which would make a large dive of itself; and, besides, the currents are troublesome during high water. [Illustration: SCORPION-SHELL. (_Pteroceras lambis._)] At the right hour the Mexicans go out in their canoes, one man of the four or five in each canoe paddling, while the rest scrutinize the bottom. It may be rocky and weed-grown, but the water is clear, and their practised eyes detect a single round oyster where you or I certainly would overlook a dozen of them. Then down a man goes and brings up his prize, with perhaps some additional ones. Sixty or eighty feet is not too deep for these adventurous divers, who will stay a whole minute upon the bottom. No food is eaten by these men on the day they dive until their labor has been done. Western Australia is another fruitful field for pearl-oysters, and until a few years ago they were taken there by native blackfellows, diving without weights or any other assistance in any water not more than ten fathoms deep. The inshore shallows have now been so cleared of shells that the only profitable industry is to go down in deep water in diving-dress and make a thorough clean-up of each “patch” where the shells seem numerous. The divers find it an interesting and curious world where they work, but one full of fright and peril. Some men who attempt it are so unnerved that they will never make a second descent. None can endure the practice long without ill health resulting; and the native Australians will never enter a diver’s dress, declining to go down where it is too deep to dive naked.