478 Mr. J. E. Gray on the Operculum 
In the paper before referred to, I stated, “The operculum agrees 
with the valve of shells in being developed on the embryo while 
included in the egg, and in increasing in size by the addition of 
new matter round the circumference of the base of the cone of 
which they are formed. They also agree in the cone being some- 
times simple and straight, and sometimes curved into a spiral 
form.” The principal difference between the operculum and the 
valve or shell of the Gasteropods consists— 
1. In the operculum having no cavity, the cone of which it is 
formed being either very much depressed, so as to become nearly 
flat or even concave, as in the annular or subannular operculum ; 
or very much compressed, forming only a spiral riband, as in the 
spiral operculum. 
2. The operculum of by far the greater number of Gasteropods 
is only formed of animal matter, so that the operculum is as if 
formed entirely of what constitutes the periostraca or drap marin 
of the shelly valves ; but the shells of some Gasteropods, as that 
of the Aplysia, Bulle, and of some land mollusks, and the valves 
-of some bivalves, as Lingula, have only a very thin shelly in- 
ternal layer, strengthening the thick periostraca; on the other 
hand, many opercula, like the generality of shells, have a shelly 
coat deposited on the inner side of the horny or periostracal coat, 
and others have the outer surface of this part, like Cyprea and 
some other genera of shells, covered with a shelly coat. 
The absence of a cavity is a difference only of degree, for the 
valves of some Gasteropods, as Umbrella for instance, are so flat 
as to produce no cavity, and thus greatly resemble the annular 
opercula of Ampullaria and Paludina, as the flat valves of some 
Calyptre are like the spiral opercula of Littorine ; but the greatest 
resemblance is to be observed in the small flat valves of Gryphea, 
Exogyra, Chama, and other genera of bivalve shells which are 
attached by one of their valves. These valves are often quite as 
flat and destitute of any cavity as the operculum of any Gaste- 
ropod ; and it is’to-be remarked that these valves exactly resemble 
a spiral operculum in shape, the remains of the ligament forming 
a spiral mark on the outer surface, showing how the valve has 
rotated on the body of the animal as the operculum rotates on 
the foot of the Gasteropods. 
Having thus shown the reasons which induced me to regard 
the operculum to be a modification of the other or shelly valve 
of a Gasteropod mollusk, I shall now proceed to show why I have 
been induced to believe that it is analogous to the second valve 
of a bivalve. 
In the Philosophical Transactions for 1833 I remarked, “ A bi- 
valve shell is composed of a dextral and a sinistral valve united 
together by a ligament. When the two valves are separated and 
