188 PECTINIBRANCHIATA.—NATICAD A. 
we can scarcely believe that it can possibly be 
withdrawn into the shell ; yet this is easily effected, 
and that so completely, that the closely-fitting 
NATICA, 
operculum is seen, shutting the aperture consider- 
ably within the margin. It is performed, however, 
if I may judge from my own experience of several 
British species, with some difficulty, and by a 
succession of efforts; and the animal when once 
forced by annoyance to retire, is often ill-disposed 
to protrude again, at least for a considerable time. 
The species of the genus are widely distributed, 
and are most numerous within the tropics. Several 
of them are of large size. Seven or eight species 
are found on the British shores, of which the finest 
is Natica monilifera. 
In this large and handsome shell, the form is 
globose, nearly as broad as long; it is strong 
but not solid, smooth and glossy, though with a 
few fie sunken lines, which mark the progressive 
growth. ‘The ground-colour is whitish, sometimes 
tinged with flesh-red, or with bay, and each whorl 
is marked along its upper margin with a line of 
oblique but parallel dashes of rich chestnut. 
