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 Gryphcea, 

 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 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 



