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. FolÂlowing 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 comÂmodious 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.
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