456 THE OPERCULUM. 



have a distinct one : and the genera Cryptostoma and Con- 

 cholepas have very small opercula, in comparison with the 

 size of their mouths, whilst the other genera allied to them 

 have their opercula nearly as large as the mouths of their 

 shells. The genus Vermetus is in this respect very remark- 

 able : most of the species have the operculum as large as the 

 mouth of the shell ; but there is one in the British Museum 

 which has an operculum very small in comparison to the size 

 of the body of the animal, and not one fourth part of the 

 diameter of the tube of shell. Some species of this genus, 

 indeed, are described as having no operculum ; and the ob- 

 servation of the above fact induces me to give credit to the 

 description, which I was at first inclined to doubt. 



But of all the variations in this particular, those of Capu- 

 lus and Hipponyx are the most remarkable : some species 

 appear always to have an operculum, which, like the under 

 valve of Crania, differs in thickness according to the form 

 and degree of exposure of the substance to which it is at- 

 tached : others, as the common Capulus hungaricus, are 

 generally without operculum, although, according to the ob- 

 servations of Dr. Turton, the last-named species sometimes 

 forms a thin support ; and there are others which, instead 

 of forming an operculum, make for themselves (as has been 

 already alluded to in this paper), a cavity in the substance of 

 the shell to which they are affixed, which is marked with a 

 lunate ridge, corresponding with the muscular scar of the 

 operculum, and doubtless occasioned by the attachment of 

 the adductor muscles to that part of the shell, which is thus 

 protected from the dissolving power of the mantle. 



