482 Gn the Operculum of Gasteropodous Mollusca. 
culum together are homologous to the two valves of a Conchi- 
ferous mollusk. 
I was also led to believe that the normal or typical form of 
Mollusca is to be protected by two valves or shells, and I was 
strengthened in this impression by the discovery that several 
mollusks which have no shell in their adult state, as the Doride, 
&c., have their newly hatched young covered with two shelly 
valves which afterwards fall off. 
With this idea, m the ‘ Synopsis’ of the British Museum for 
1842, p. 50, I observed, “ By far the greater number of these ani- 
mals (Mollusca) are provided with two of these shells or valves, 
which are often nearly alike in size and form, and are hence called 
bivalves, as the shells of the Conchifera, where one of the valves is 
placed on each side of the body and they are united together by a 
ligament. In others, as those of the Brachiopods, the two valves 
are separate, one on the upper surface or back, and the other on 
the under surface of the body. In others, as in the shells of 
Gasteropods, the two valves are so unequal that the smaller 
merely acts as a lid to close the mouth of the larger one when 
the animal is retracted into it ; hence it has been called an oper- 
eulum. This smaller valve or operculum is generally cartilagi- 
nous, either wholly formed of animal matter, or strengthened by 
a quantity of calcareous matter deposited on one or both of its 
surfaces ; sometimes this valve is altogether wanting, especially 
in those genera which have an expanded mouth compared with 
the size of the remaining shell. In the bivalve Conchifera and 
Brachiopoda the two valves are usually nearly equal-sized, and 
regular in position. On the contrary, in the Gasteropods the valves 
are unequal, and placed more or less obliquely with regard to the 
axis of the elongated body of the animal. 
.If this theory is correct, the operculum should afford an im- 
portant character for the distinction of families and genera; and 
this has proved to be the fact. 
In 1821 I first drew attention to the very good character which 
it afforded, not only for the distinction of genera, but also for 
the division of the genera into larger groups. In my papers pub- 
lished in the Zoological Journal and in the Philosophical Trans- 
actions, I collected together the results of my observations on 
their structure, formation and growth,:and their importance to 
the ceconomy of the animal. More recent. examinations have 
only strengthened my conviction, that they afford quite as import- 
ant characters for the division of families and genera as the shell 
of the Gasteropods themselves, and that to neglect them in the 
description of the genus or species is quite.as rational as to de- 
scribe only the single valve of a bivalve shell. If this is the case, 
no specimen of an operculated univalve, which is not accom- 
