The lining is constructed of thin waves of transparent calcium carbonate set in animal tissue of great tenuity. This is the mother-of-pearl, and the gem differs from it only in its more or less rounded and independent formation. The plates of which the lining is composed lie almost parallel to the plates of the epidermis. They are bent a little toward the interior at the inner surface of the shell, but the general sectional structure of a shell, cutting from the umbo to the lip, is fairly represented by that stem of the letter X which extends from the right upper to the left lower, the diagonal line representing the middle shell; the horizontal lines at the extremities show the general trend of the epidermis and the nacreous lining. The diagonal trend downward is from the epidermis toward the boss-end of the shell. The nacreous plates, or mother-of-pearl, unlike those of the middle shell of the nigger-head, cannot be easily separated. On cutting them across the grain they appear as distinct and separate strata and show dividing lines, yet the mass is compact to a great degree. Upon being broken, these strata separate only at the edges, so that the entire set usually breaks diagonally, showing a small strip of the surface of each plate along the broken edge and forming a series of ragged edge steps. These plates or strata are composed of a great many very thin waves following one upon the other, and thereby producing series of fine, irregular lines upon the surface which, though trending generally in straight lines, curve and twist about as do the edges of water waves, when they run up on the sands of the sea-shore. It is the lapping of these thin transparent waves, and the minute undulations of the layer edges reflecting through the transparent plates, which produce the soft luster peculiar to the linings of the shells and the surface of pearls, and which is known as "pearly." The wave edges do not usually produce iridescence, but if the waves are very thin and transparent the undulating lines of many under waves following close upon each other appear on the surface, under the microscope, as dark lines when the light is passed through the skin, or silvery lines if the light be thrown upon it from above; to the naked eye this becomes the tempered brilliancy of the pearl's orient. Under the microscope these waves appear to be constructed of minute hexagonal plates or prisms set in animal membrane. A set of waves forming a plate, when broken at right angles to the trend of the wave, shows under the microscope a rough irregular edge, and the small plates of which they are composed sometimes appear separated individually from the mass though more often they are dislodged in clusters or strips. Broken with the trend of the wave edges, the plate breaks diagonally in steps with undulating edges, which correspond in appearance with the successive underlying waves as they are seen through the surface under the microscope.