ON MOLLUSCA. 175 
the molecules, brings them to unite under an inorganic form, 
and to crystallize; and then the testaceous spoils of the mol- 
lusca have a greater or less tendency to disappear, and to form 
calcareous masses by their agglomerations, and especially by 
that of their pieces or detritus, which constitutes the forma- 
tions of the shelly limestone. 
According to all that we have now said, the shells present 
very considerable differences, according to the age of the ani- 
mal to which they belong; and these differences sometimes 
prevail on the form of the aperture, and especially on that of 
the right edge of the univalve shells. 
They also present differences according to the sexes in the 
dioecious groups, that is, where the male sex characterizes one 
individual and the female another, as we shall see by and by. 
We shall now cast a glance on another production of the 
skin, the use of which is to render the protecting apparatus 
still more complete, and which is designated under the name 
of operculum, because it serves to close more or less com- 
pletely the aperture of the shell, even at its orifice, or more or 
less deeply. Some authors, and among others Adanson, have 
regarded it as the analogue of one of the valves of a bivalve 
shell; but this is clearly incorrect, for its position, in relation 
to the body of the animal, indicates no analogy of the kind. 
The two valves of a bivalve are placed one on each side of its 
body, except, perhaps, in the palliobranches, while in the oper- 
culated mollusca the shell alone, dependent on the mantle, 
constantly occupies its dorsal face, and the opercle has never 
any connexion but with the upper dorsal face of the foot, 
sometimes at the angle of its junction with the pedicle of the 
body, rarely at its posterior extremity, and most frequently in 
its middle part. It is evidently the production of the skin 
which covers the foot. ‘This production is, without doubt, an 
excretion of calcareous or corneous matter; but how a plane, 
oval, or circular surface produces a substance which rolls 
