organic matter which serves as a uniting medium. In nacreous or
pearly shells we find an intimate association of the two textures,
often in the form of alternating layers of very thin animal membrane
and carbonate of lime. If digested in weak acid the calcareous
ingredient of such a shell is slowly removed in solution, while the
membranous residue retains with fidelity the original form of the
shell. The shell is secreted by the soft external integument of the
mollusc known technically as the "mantle," since it forms a kind of
cloak, enveloping the viscera or internal organs. The edge, and indeed
the general surface of this delicate membrane, separates calcareous
matter from the food of the mollusc and, from the surrounding medium,
and thus slowly builds up the texture ot the shell. In some molluscs
the shell is formed ol only a single piece, or valve, and hence they
are termed Uni-valves. The snail, the whelk, and the nautilus
are familiar examples of such uni-valved mollusca. But in other cases
the shell is composed of two parts, whence they are termed Bi-valves. It is to this group, as previously stated, that all the ordinary Pearl-bearers belong.
Marine Pearls.
The great class of the Lamellibranchiata, or Conchifera, including all the common bivalves,
