INTRODUCTION. 



29 



In some cases the columella is swelled into a varicose mass ; 

 as in Oliva, Ancillaria, &c. ; it is then described as tumid or 

 varicose. It is sometimes tortuous, and sometimes straight, 

 and is susceptible of many variations, too minute and par- 

 ticular to be described in this part of the work. 



Columellar Lips. 



Fig. 60, Melo, obliquely plaited; 61, Turbinellus, horizon- 

 tally plaited ; 62, Ancillaria, varicose, tortuous; 63, Na- 

 tica, straight. 



OPERCULUM. 



The aperture of many species of shells remains constantly 

 open ; but in a great number of species it is occasionally closed, 

 whenever the animal is retracted within the shell, by a calca- 

 reous or horny piece called the operculum. This must be dis- 

 tinguished in the first instance from another kind of calcareous 

 covering, which in some univalve shells serves to close the aper- 

 ture during a certain portion of the year. This piece, named the 

 epiphragm, although hardened and shelly in appearance, is no 

 real part of the animal or of the shell ; being only a secretion 

 temporarily hardened, for the purpose of defending the animal 

 from external influences during the hibernating or torpid 

 season, to be dissolved when that season is at an end. On 

 examining this piece, it will be observed that it is not formed 



