stem
of the letter X which extends from the right upper to the left lower,
the diagonal line representing the middle shell; the horizontal lines
at the extremities show the general trend of the epidermis and the
nacreous lining. The diagonal trend downward is from the epidermis
toward the boss-end of the shell.
The
nacreous plates, or mother-of-pearl, unlike those of the middle shell
of the nigger-head, cannot be easily separated. On cutting them across
the grain they appear as distinct and separate strata and show dividing
lines, yet the mass is compact to a great degree. Upon being broken,
these strata separate only at the edges, so that the entire set usually
breaks diagonally, showing a small strip of the surface of each plate
along the broken edge and forming a series of ragged edge steps.
These
plates or strata are composed of a great many very thin waves following
one upon the other, and thereby producing series of fine, irregular
lines upon the surface which, though trending generally in straight
lines, curve and twist about as do the edges of water waves, when they
run up on the sands of the sea-shore. It is
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