the
wave edges, the plate breaks diagonally in steps with undulating edges,
which correspond in appearance with the successive underlying waves as
they are seen through the surface under the microscope.
Although
distinct dividing lines between the plates appear when a sectional cut
is made across the grain, there is no indication of a division between
the waves which make up the plates, and there is no apparent difference
in the structure or compactness at the junction of the plates though a
clean division can only be made there. It would appear, therefore, that
the plates mark intervals in the process of construction and that the
animal tissue is somewhat thicker between the plates than between the
waves of which they are composed, where the formative process has been
continuous.
In
all parts of the shell, the calcium carbonate takes the hexagonal form:
in the nacre, as thin waves composed of hexagonal faces, and in the
middle shell and epidermis, as plates of hexagonal particles grouped
as hexagonal prisms whose terminations form the front and back of a
plate. All the parts show a similar plan of
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