THE OPERCULUM. 



447 



the size of the operculum, and is situated on its right side : 

 it is cordate and marked with oblique rugose grooves, and 

 with a strong central ridge, which is continued in the form 

 of a rib down the middle of the under side of the free part 

 of the operculum. In some species the posterior edge of 

 the operculum is serrated. 



In both the annular and subannular divisions, the disk 

 to which the operculum adheres is formed entirely of the 

 muscle of attachment, and of a membranaceous fringe by 

 which its edge is surrounded. This fringe is free from the 

 back of the foot and is widest posteriorly : it doubtless 

 secretes the coat of the layer of growth, and the deposit 

 which borders the edge of the scar, whilst the muscle itself 

 is provided with the means of secreting the proper coat of 

 the scar. 



The spiral opercula are so called because the elongated, 

 cartilaginous or shelly, compressed, riband-shaped cone of 

 which they are formed is twisted into a spire, of few or 

 many whorls (Fig. 81). In these, the new matter by which 



Fig. 81. 



they increase in size is deposited only on the extremity of 

 the last whorl, which in spiral opercula may be regarded as 

 analogous to the mouth of spiral shells, as the entire circum- 

 ference of the annular opercula, with their subcentral apex, 

 is analogous to the edge of the base of a Patella. The lines 

 of growth are marked by curved concentric lines extending 

 transversely across the whorls. Opercula of this character 

 vary much in the number of their whorls : when the whorls 

 are few in number the cone increases rapidly in size, and 

 the operculum is of an ovate shape, with the nucleus ap- 

 proximated to the left extremity ; when the whorls are more 

 numerous, the cone increases in size more gradually, the 

 nucleus is central, and the operculum is more orbicular. In 

 all these cases the edge of the extremity of the last whorl 

 is constantly anterior, that is to say, when the operculum is 

 in the mouth of the shell, this part of it is directed towards 

 the inner lip ; when the operculum is ovate and of few whorls 



