In a general way therefore, the animal deposits the best of its secretions about itself and pushes out to the outer extremities, the coarser elements which are fitted to preserve the finer parts of the shell, as the finer parts of the shell are fitted to protect the delicate organism which they enclose. The building of the shell is done by a membraneous covering of the fish which entirely envelops the body and is attached to the shell a short distance from the inner edge, leaving a rim of membrane free around the fish and the edges of the two valves. This membrane is called the mantle. It extracts lime from the water, and at different parts exudes modified solutions of it mixed with animal tissue, suitable for the construction of the various parts of the shell. The exterior of the shell or epidermis consists of conchiolin, an organic compound. It is a horny-looking substance, and in the large salt-water shells and in most of the fresh-water mussels, the nigger-head of the Mississippi Valley especially, it appears to the eye as a series of extensions, sometimes terminating in ridges, which curve about the umbo and spread to the edge of the shell, each extension coming from under the one preceding. In some varieties it is attached as an excrescence to the prismatic formation immediately under it, and may be easily detached in thin flakes: a rusty black in some, brownish-yellow in all on the inner surface and in some on the outside. The substance is generally opaque, but contains spots of which some are translucent, resembling horn or amber, while others are more transparent, similar in formation to the inner parts of the shell. In most of the marine and fresh-water varieties, unlike the nigger-head, the conchiolin exterior does not easily flake off. In these the outer shell is composed of wave-like plate extensions, superimposed one upon the other recedingly from the lip to the umbo as in the others, but without the ridges, the plates being flat and the edges more irregular. These extensions are formed of a number of horizontal composite plates, which penetrate the shell to the mother-of-pearl. Not only may they be separated into thinner horizontal plates, but they divide vertically into prisms. Under the microscope the edge of a composite plate appears as a number of prisms placed side by side lengthwise across the plate edge, but showing dark, intersecting lines through the series where they divide as plates.