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
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