THE OPERCULUM. 285 



inflected, or turned inwards. Some outer lips are denticu- 

 lated, or studded with small teeth; some dentated, when 

 the teeth are larger. Some are expanded into flattened lobes, 

 or alated ; others drawn out into long lobes, when they are 

 said to be digitated. 



The columella, or columellar lip, in some instances lies 

 upon, and is lost, as it were, in the last whorl; in other in- 

 stances it is free and independent, standing out in a com- 

 plete ridge separate from the body of the shell. The lower 

 part of it is perhaps more properly named the columella 

 wdien it forms a solid axis on which the shell turns. Many 

 of the columellse are ornamented by oblique or transverse 

 folds : these are remarkably elegant in the species of Mela 

 and Cyniba: in Turhinella they are more horizontal. 



There are cases in which there is a hollow, or umbilicus, 

 behind the inner lip ; and some where this hollow is con- 

 tinued up to the very apex of the shell. The usual means 

 of expressing this is to say, ^^axis* perforated. ''■' 



THE OPERCULUM, 



As before observed, is a kind of accessory valve, or door, 

 used by the animal in closing the aperture of his shell, after 

 snugly enscoi]cing himself in his quarters. It is formed of 



