On the Operculum. 211 



out the presence or absence of this organ, which is so important in 

 the habits of the animal, and if present, shews by the distance Avhich 

 it extends towards the anterior part of the shell, the length of the 

 tubes. These characters certainly afford great advantages to a 

 conchologist, in as much as they enable him to understand the 

 habits of the inhabitant by the mere inspection of the shell. 



§ 5. On the Operculum. 



The aperture of many of the spiral univalve shells is more 

 or less completely closed, when the animal has retreated within its 

 habitation, by a shelly or horny substance, which is denominated 

 the Operculum ; and which is attached to the back of the hinder 

 end of the foot of the Ctenobranchous, and some of the Pneumo- 

 nobranchous Mollusca, by which the shells are formed. It closes 

 the mouth by the contraction of the hinder portion of the muscle 

 that is attached to the columella, the anterior portion of which 

 retracts the head, and the hinder part being longest, the head is 

 first enclosed. Little attention has hitherto been paid to the 

 structure of this part, which has generally been regarded only 

 with reference to its presence or absence, as a mark of generic 

 difference ; but as the nature of its structure and its consistence 

 will be found capable of assisting greatly in the arrangement of 

 such of the Mollusca as are provided with it, I shall briefly offer a 

 few remarks on its formation. The opercula are formed in the 

 same manner as shells themselves*, by the deposition of successive 

 layers on the internal surface of their margins; and are divided 

 by the manner in which these layers are deposited (as exhibited 

 by their concentric stride, or striaj of growth) into two groups. 

 In the first of these groups, or the Annular Opercula^ which are 

 analogous to the symmetrical shells, the nucleus is more or less 

 central, and the increase is by the deposition of new matter on 

 the whole edge, thus exhibiting ring-like concentric striae. The 

 most distinct t)i)o of this formation is where the nucleus is nearly 

 central, as in the Paludina;, where the Operculum is horny ; and in 



* This is peculiar as hcitig a shelly part, not formed on the mantle, but se- 

 creted by a peculiar apparatus, which renders it not impossible that Ilippoiiix 

 should belong to the Cochlcophorous and not to the Brachiopodous Mollusca. 



