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
137
