NEKITINA. 
45 
the Nerites is horny, covered on both sides with a 
hard shelly coat. The position of the horny oper« 
culum is shown by a groove in the edge between 
the two coats ; and if a knife is inserted, the coats 
can be separated from the operculum. 
As the periostraca is essential to the structure of 
the shell, and is always present, some shells being 
formed of scarcely any thing else, so it is with the 
operculum, the horny part similar to the periostraca 
of shells being always present, and forming its es- 
sential part, and a shelly coat being in some in- 
stances added to the outer surface, as in Turbo and 
Phasianella^ or to the inner surface, as in this genus, 
in which the horny part is very thin and scarcely 
visible, except where the shelly coat is very thin, as 
at the flexible edge. 
These animals absorb the septa which separate the 
whorls of the spire, when they have arrived at their 
full size, so as to allow more room for the spiral 
body, without increasing the size of the shell ; and 
this can be done without endangering the strength 
of the shell, as only a very small part of the whorl 
is exposed on the surface. A similar absorption is 
to be observed in many AuriculidcB^ and to a less 
extent in the CdneSy where the septa are only reduced 
in thickness. (See Phil, Trans, 1833, p. 798.) 
This absorption is only superflcial, and produced 
by that portion of the surface of the mantle which 
lies close to it,* and is not to be confounded with the 
absorption of the bones of vertebrated animals, where 
it is produced by vessels which ramify in the sub- 
stance of the bone, and which are accompanied by 
