of Gasteropodous Mollusca. 477 
as in Adlanta, sometimes very reduced ; on its middle the byssus 
is secreted, whose filaments when present are always anes into 
a dise-like operculum. 
“ The side part of the lobe is sometimes produced into cirri, 
as in Rissoa, Lacuna, &c., sometimes into a large extensible mem- 
brane capable of covering the whole shell.”—Lovén, Ofvers. 
Kongl. Vetensk. Akad. Fork. 1847. 
In a paper on the Structure of Shell in the Philosophical 
Transactions for 1833, I showed with considerable detail that the 
operculum of the Gasteropodous Mollusca, like the shelly valve 
of those animals, 
1. Is developed on the embryo long before it is hatched. 
2. That it is placed on and covers a peculiar part of the body, 
which bears the same relation to it as the part of the body called 
the mantle bears to the part usually called the shell of these 
animals ; and it is formed and increased in size by an opercular 
mantle in the same way as the shells are. 
3. That the operculum is more or less conical, and is increased _ 
in size by the addition of new matter to the inner surface, and 
especially to the surface near the margin; the new matter either 
forming more or less complete rmgs round the nucleus (or first- 
formed part), when they are called annular, and are homologous 
to the simply conical shell, as the Patella; or else the new matter 
is deposited almost entirely on one edge of the nucleus, when the 
operculum forms a more or less elongated cone, which, when long, 
is spirally twisted round an imaginary axis (like a spiral shell), 
the broad part of the cone being next the edge of the opercular 
mantle which secretes the new matter for enlarging its size, as 
the mouth of the shell is on the outer edge of the mantle of the 
univalve shell. 
4. That the operculum is attached to the animal by means of 
one or more muscles, which, as in the bivalve shell, pass from 
the larger valve or shell to the smaller one or operculum. 
5. The operculum as it increases in size is gradually moved 
on the end of the muscle; the many-whorled operculum of the 
Trochi revolves as many times on the end of the muscle, as the 
many-whorled spiral shell turns on its imagimary axis. 
6. The operculum is moulded on the opercular mantle, and 
is often lined internally with a shelly coat like a shell ; and some- 
times, like the shell of the Cowries, it has its outer " surface co- 
vered with a shelly coat deposited by some special development of 
the opercular mantle especially destined for the purpose, as is the 
case in the Cowries and some other shells. 
From these observations it would appear, that the operculum 
has all the characters of the part of the animal which has been 
usually considered as the shell. 
