Although distinct dividing lines between the plates appear when a sectional cut is made across the grain, there is no indication of a division between the waves which make up the plates, and there is no apparent difference in the structure or compactness at the junction of the plates though a clean division can only be made there. It would appear, therefore, that the plates mark intervals in the process of construction and that the animal tissue is somewhat thicker between the plates than between the waves of which they are composed, where the formative process has been continuous. In all parts of the shell, the calcium carbonate takes the hexagonal form: in the nacre, as thin waves composed of hexagonal faces, and in the middle shell and epidermis, as plates of hexagonal particles grouped as hexagonal prisms whose terminations form the front and back of a plate. All the parts show a similar plan of construction, _i.e. _, separable plates composed of thinner plates more compacted together, and these in turn of infinitesimal hexagons of calcium carbonate; full plates, component plates, and particles, all alike surrounded by animal tissue. The shell is built up of secretions from the water in which the oyster lives, made by the mantle, a membraneous covering of the fish. The function of this mantle, in part, is to obtain from the water the elements required and exude it at different parts of its folds in the various forms required for the several parts of the shell. The necessary lime exists in the surrounding water and is supplied sometimes by the calcareous beds upon which the oysters grow, and in other cases by surrounding vegetation. In all mother-of-pearl oysters and the fresh-water mussel unio, the lining is usually quite thick, but in some pearl-bearing species having small, frail shells, it is, though beautiful, too thin to be of use. In the meleagrina, this nacreous lining lies in the interior of the shell like a congealed pearl wave, the smooth even rim following the curve of the shell about an inch to an inch and a half within the jagged edge of the epidermis, as shown in the Manilla shell illustrated herewith, in which the lip, usually trimmed off for commercial purposes, is preserved. The lining of the meleagrina is not as iridescent as that of the thin shell varieties. Thus the shell is being constantly enlarged at the edge, by a deposit of the exudations of the mantle; conchiolin for the epidermis outside, lime for the prisms and inner layers of transparent plates, until the shell has attained its full growth in size, after which some varieties continue to lay on nacre only. [Illustration: MANILA PEARL-SHELL WITH THE LIP CONSERVED]