GASTEROPODA. 
13 
in an abnormal direction, the direction of the oper- 
culum is also changed. 
9. The opercula are repaired, when injured or 
partly broken off, in the same manner and by the 
same means, and when repaired offer the same ex- 
ternal appearance, which shells do under similar cir- 
cumstances. See Ann, and Mag. N. H. 1850, v. 
476. ; and 1854, xiii. 419. , 
The principal difference between the operculum 
and the valves or shells of the gasteropod con- 
sists — - 
1. In the operculum having no cavity. The cone 
of which it is formed is either very much depressed, 
so as to be nearly flat or even concave, as in the 
annular or subannular operculum, or very much 
compressed, forming only a spiral band, as in the 
spiral operculum. 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 operculum of Ampullaria^ and the flat valves 
of some CalytrcB are like the subspiral operculum of 
Littorina^ but the greatest resemblance is to be ob- 
served in the small flat valves of some GryphcBa^ 
Exogyra, Cliama^ 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 gasteropod ; and it 
is to be remarked that these valves exactly resemble 
a spiral operculum in shape, the remains of the liga- 
ment forming a spiral mark on the outer surface, 
showing how the valve has rotated on the body of 
