The Pearl, its story, its charm, and its value

by Wallis Richard Cattelle

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.

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.

The effect is similar to that made by a pack of cards set diagonally so as to spread the edges sufficiently to show the merest trifle of the faces of the cards between the edges. The arrangement of these plates, not only produces a series of fine lines curving about the umbo, but, as the edges are slightly irregular, another series of fine lines cross the others at right angles, radiating from the umbo. This doubly striated surface, by interference, produces an iridescence more full of color than the mother-of-pearl of any but the thin-shelled varieties.

Though similar in construction, these plates differ from those of the epidermis. In some respects they suggest a transitional stage between the outer and inner shell. A plate, as it separates from the series and which appears as one line in the striated surface of plate edges, is in reality a number of very thin plates, or waves, so welded together that they cannot easily be separated. In this and the presence of fine surface lines marking the wave edges, they resemble the nacreous plates.

The composite plate is opaque, but when split so that light can penetrate there appears on the face, markings similar to the unexposed portions of the Venezuelan epidermis plates only the hexagonal faces are very much smaller and less distinct. So also the edge of the composite plate appears as series of prisms crossing it from face to face on the plate, in sets which show plainly, lines marking the juncture of the individual plates or waves. Although the individual plates or waves, can only be separated with great difficulty, together, as composite plates, they can be flaked off from the shell very easily, and they crumble and break into fragments under slight pressure. The component plates or waves are very thin, and appear under the microscope as white and translucent planes marked by outlines of the prism faces.

The inner series of these plates as they near the nacreous lining become harder and more compact, and incline more and more to a horizontal position, so that at the point where they abut upon the nacre it is not easy to distinguish them from the nacreous plates. At the thinner end of the shell, about the edges, the plates are all of this nature. They grow more friable and chalky as they incline to the perpendicular, where the series are more numerous and are situated at the thicker part of the shell about the umbo.