MOI/LUSCA. 7 



fire, these shells crack in various directions, but exhibit no 

 trace of a scaly structure. By careful management with 

 the file, the shell may be separated into a central layer 

 contiguous to the skin, and a peripheral layer, both similar 

 in structure, though frequently differing in colour. The 

 shells exhibiting such characters have been termed porcel- 

 laneous, from their dense structure, and the fine polish 

 which their surface presents. The formation of shells of 

 this kind must take place in a different manner from those 

 of the first kind which we have noticed. 



If we attend to the form of a young shell belonging to the 

 genus cypraea of Linnaeus, we may perceive that an addition 

 of shelly matter to the margin of the aperture, in the man- 

 ner in which it is applied in other shells, would not enlarge 

 the cavity, but completely close the aperture. The increase 

 of the shell, (accompanied with a corresponding increase of 

 its inhabitant), must take place, therefore, either by ab- 

 sorption of the accumulated shelly matter of the mouth, and 

 an elongation in the direction of the greatest curvature of 

 the shell ; or the old shell must be thrown off, and a new one 

 produced, suited to the size of the animal. The former sup- 

 position has not been entertained, the latter is now generally 

 received by naturalists, though there are a few individual^ 

 unwilling to adopt such a view of the matter. The inner 

 coat of such shells appears to be a transudation from the 

 body of the animal, and the outer one to be laid on the sur- 

 face by the loose reflected lobes of the cloak. In many other 

 shells, portions of matter, more compact than the other parts, 

 may be observed spread on the pillar, and applied to the 

 margin of the mouth by a similar process. Mr. Platt, in 

 support of Reaumur's opinion, that shells are formed by jux- 

 taposition, against the objections of Mr. Poupart, (PhiL 



