MOLLUSCA. 403 



Porcellatwous, from their dense structure, and the fine po- 

 lish 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 Cyprsea of LINN&US, we may perceive that an 

 addition of shelly matter to the margin of the aperture, in 

 the manner 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 in- 

 crease of its inhabitant), must take place, therefore, either 

 by absorption 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 

 supposition has not been entertained, the latter is now gene- 

 rally received by naturalists. 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 surface by the loose re- 

 flected lobes of the cloak. In many other shells, portions 

 of matter, more compact than the other parts, may be ob- 

 served 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 juxtaposition, 

 against the objections of Mr POUPART *, erroneously consi- 

 ders the different sizes of the Cypreae as depending on the 

 thickness of the shell increasing according to age, without 

 admitting a corresponding increase of the dimensions of the 

 contained animal, or cavity for its reception. 



The shells of the first kind which we have noticed, from 

 being formed of cones or layers applied to the inner edge of 

 the margin, and extending beyond it, have an imbricated 



* Phil. Trans, vol. liv. p. 43. 



