Mother-of-pearl is the hard, silvery, internal layer of several kinds of shells, especially oysters, the large varieties of which in the Indian Seas secrete this coat of sufficient thickness to render the shell an object of manufacture. The genus ot shell-fish, Pentadinrz, furnishas the finest pearls as well as mo-ther-ot-pearl: it is found round the coasts of Ceylon, near Ormus, in the Persian Gulf, at Cape Comorin, and in some of the Australian seas. The dealers in pearl- shells consider the Chinese from Manilla to be the best; they are fine, large, and very brilliant, with yellow edges. Fine large shells of a dead white are supplied by Singapore. Common varieties come irom Bombay and Valgraiso, from the latter place with jet black edges. South Sea pearl-shells are common, with white edges. The beautiful dark green pearl-shells called ear-shells or sea-ears, are more concave than the others, and have small holes round the margin; they are the coverings of the Haliotis, which occurs in the Californian, South African, and East Indian Seas. In the Indian collection of the Great Exhibition, specimens of the finest pearl shells were shown, such as the Meleagrina margari-tifera, Haliotis gigas, Haliotis iris, and a large species of Turbo, which shells are known in commerce as flat-shells, green snail-shells, buffalo-shells, Bombay shells. Messrs. Faunt-leroy and Mr. Banks had also some fine collections. The latter gentleman states that the shore of the Sooloo Islands affords the finest shells. The beautiful tints of the mother-of-pearl depend upon its structure; the surface being covered with a multitude of minute grooves, which decompose the reflected light. Sir David Brewster, who was the first to explain these chromatic effects, discovered, on examining the surface of mother-of-pearl with a microscope, " a groved structure, like the delicate texture of the skin at the top of an infant's finger, or like the section of the annual growths of wood as seen upon a dressed plank of fir. These may sometimes be seen by the naked eye; but they are often so minute that 3,000 of t-iera are oflataiBed in an inch." It is remarkable that tBese iridescent hues can be communicated to other surfaces as a seal imparts its impress to wax. The colors may be best seen by taking an impression of the mother-of-pearl in black wax; but " a solution ofgumarabic or isinglass, when allowed to indurate upon a surface of mother-of-pearl, 4akes a most perfect impression Irom it, and exhibits all the communicable colors in the finest manner, when seen either by reflection or transmission. By placing the isinglass between two finely-polished surfaces of mother-of-pearl, we obtain a film of artificial mother-of-pearl, which, when seen by single lights, such as that of a candle, or by an aperture in the window, will shine with the brightest hues." It is in consequence of this 1 amellar structure that pearl shells admit of being split into laminae for the handles of knives, for counters, and for inlaying. Splitting, however, is liable to spoil the shell, and is therefore avoided as much as possible. The different parts of the shell are selected as nearly as possible to suit the required purposes, and the excess of thickness is got rid of at the grindstone. In preparing the rough pearl-shell, the square and angular pieces are cut out with the ordinary brass-back saw, and the circular pieces, such as those for buttons, with the annular or crown-saw, fixed upon a lathe-mandrel. The pieces are next ground flat upon a wet grindstone, the edge of which is turned with a number of grooves, the ridges of which are less liable to be clogged than the entire surface, and hence grind more quickly. If the stone be wetted with soap and water it is less liable to be clogged. The pieces are finished on the flat side of the stone, and are then ready for inlaying, engraving, polishing, c. Cylindrical pieces are cut out of the thick part of the shell, near the hinge, and are rounded on the grindstone preparatory to being turned in the lathe. The finishing and polishing are described in the third volume of Mr. Holtzapffel's excellent work on " Mechanical Manipulatioa" Counters, . silk-winders, c, are smoothed with Trent wk sand or pumice-stone and water on a bufl wheel or hand-polisher, and are finished with rotten-stone moistened with sulphuric acid, which developes finely the striated structure of the shell. For inlaid works the surface is made flat by filing and scraping ; then pumice stone is used, and after this putty-powder, both on buff-sticks with water; and the final polish is given with rotten stone and sulphuric acid, unless tortoise-shell or some other substance liable to be injuiiously affected by the acid be present in the inlay. In turned works fine emery paper, rotten stone and acid or oil are used. The pearl handles for razors are slightly riveted together in pairs, then scraped, sand-buffed on the wheel with Trent sand and water; thirdly, gloss-buffed on the wheel with rotten stone and oil, or sometimes with dry chalk rubbed on the same wheel; and fourthly, they are handed up, or polished with dry rotten stone.