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the size of the lost part, not quite as thick as the original portion, and 
formed of rather irregular horny plates; the smaller or Fig. 1, 
first-formed portion being in the centre of the broken 
line, so that the restored part bears some similarity to 
the annular operculum of a Paludina. This restoration 
is exactly like that which would have taken place in a 
shell under similar circumstances, and is a further 
proof of the truth of the theory which I have long 
advocated, that the operculum is a rudimentary valve, 
and is homologous to the second valve of the Bivalve 
Mollusks. 
Tn examining two specimens of Plewrotoma babylonica, preserved 
in spirits, with the opercula attached, I was much surprised to 
observe that the opercula of the two specimens were exceedingly 
different in structure and belonged to two distinct modifications of 
that valve, one (fig. 2) being subannular, with the nucleus apical, 
like the other species of the genus, and the other (fig. 3) annular, 
with the nucleus subcentral, somewhat like the operculum of Paludina. 
Fig. 3. 
The examination of the restoration of the lost half of the operculum 
of the Fusus before referred to has solved the difficulty, and I have 
no doubt that one of these animals had by some accident lost its 
operculum, and that it had gradually restored it ; commencing, as in 
the case of the restored part of the operculum of the Fusus, by a 
small nucleus in the centre of the opercular mantle, on the back of 
the foot, and gradually adding new layers round the edge of it, until 
it formed an annular operculum nearly of the size of the original, but 
differing from it in shape, being less acute in front and nearly similar 
in form at the two ends. A more minute examination has strength- 
ened this theory, for the operculum of this specimen is less regularly 
developed than is usual in the annular operculum of the kind, and 
is much thinner than the normal operculum of the genus, as is the 
case in both these particulars with the restored part of the operculum 
of the Fusus. 
This change in the formation of the operculum when it is repro- 
duced, is just what one might have expected. The animal, when it 
has to form its operculum at its birth, begins its formation at the 
tip, and increases its size, as the animal requires a larger operculum 
for its protection, by the addition of new layers to the outer edge of 
its larger and last-formed end: but when it has to reproduce this 
organ, the opercular mantle having reached a certain size, it proceeds 
