MOLLUSCA. 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 porceé- 
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 ofa young shell belonging tothe 
genus cyprea of Linneeus, 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 anew 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 individuals 
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 tobe 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, (Phi. 
