506 THE CHAMBERED NAUTILUS. 
service in the arts, furnishing employment to many hundreds of workmen. 
As the shell of these creatures consists of one piece or valve only, they are 
sometimes termed Univalves, in contradistinction to the oysters, mussels, 
scallops, and similar shells, which are termed Bivalves in allusion to their 
double shell. : 
There is a structure belonging to these animals which must be described 
before proceeding further, inasmuch as 1s shape and comparative dimenr 
sions often afford valuable indications by which a species, or even a genus 
may be distinguished. This structure is called “onercnlum,” and its use, 
CHAMBERED NAUTILUS.—(ivaucteus £vMpsttas.) 
when fully developed, is to close the aperture of the shell when the animal 
has withdrawn itself into the recesses of its home. 
The operculum can be well seen in the water-snails, where it attains its 
full size, and exactly fits the opening which it is intended to protect. The 
material of which the operculum is essentially composed is a horny substance, 
but in some species the horn is strengthened by layers of the same nacreous 
matter which lines the shell, and becomes so thick and heavy, that when 
found separate from its owner it is often mistaken for some species of shell. 
The operculum is very variable both in its form and comparativesdimensions, 
and even in its presence or absence. Sometimes it is circular, like a flat 
