These prisms appear on the face of the plates as translucent hexagons, separated by dark lines like a tessellated floor, and under a powerful microscope are seen to be composed of similar smaller particles, also joined together by a binder of tissue. The exposed parts of the epidermis plates, forming the outer skin of the shell, are more dense than the unexposed portions; the hexagonal dividing lines are thick and blurred, and the faces are almost opaque, whereas in the unexposed parts, the faces are translucent and the hexagonal markings are clear and fine. Though constructed in the same way throughout, these plates appear to follow the general plan of shell construction, the preponderance of calcium carbonate in the interior parts gradually changing to an excess of organic matter as they become exposed to form the outer part of the shell. The outer shell is in some varieties of a brownish-yellow with radiating fan-like markings of a deeper tint or red; in others, dark gray and brown to almost black. Immediately under the surface, the plates become lighter in color, and finally almost white as they approach the nacreous interior. In all varieties the outer plates lie almost parallel with the extension of the shell, so that, lapping each other as they do, the outer contour of the shell is raised by a series of low steps from the edge to the umbo. These plates appear to have been superimposed one upon the other. On the contrary, they are added on the under side. Starting from the umbo, which is the oldest part, the shell is enlarged by the addition of a succession of plates from beneath, each series extending a little beyond its predecessor, the rough conchiolin fringe at their extremities forming the outer covering of the shell. Following the growth of the epidermis, the shell and the lining are also extended and built up, so that the entire shell is constantly pushed to dimensions necessary for the proper and commodious housing of its growing tenant. Under the thin coat of epidermis on the Unio nigger-head, is a stratum of prism plates similar to the outer plates of the Venezuelan oyster. The prism faces are however smaller and the organic intersections are thicker and darker. Immediately under and abutting, is another series of plates which penetrate the shell almost horizontally at the lip end, to the lining; diagonally at the thick part of the shell near the umbo to another series of the same kind. Here, owing to their diagonal set, upon peeling off the epidermis and the epidermis plates, the edges appear as a series of fine lines curving about and spreading out from the umbo. The plates set outward, away from the umbo, from the lower or inner edge.