STRUCTURE OF SHELLS. 211 



When porcellaneous shells are subjected to the 

 solvent action of acids, the animal matter in their 

 composition offering but little resistance, there is a 

 considerable and long continued effervescence. The 

 solution of the carbonate of lime proceeds rapidly, 

 in consequence of the speedy disintegration of the 

 animal substance, which is broken up, and partly 

 dissolved. The remainder is reduced to minute 

 fragments, which subside in the form of flakes or 

 scales to the bottom of the fluid.* 



The difference between the textures of these two 

 kinds of shell is farther illustrated by the impres- 

 siop made upon them by tire. Porcellaneous shells, 

 when exposed to a red heat, give out neither smell 

 nor smoke : they lose, indeed, their colour, but re- 

 tain their figure unaltered. Membranous shells, on 

 the contrary, emit a strong fetid odour, and become 

 black; after which the plates separate, and the 

 structure falls to pieces. 



This variety in the composition and structure of 

 different kinds of shell is accompanied by corre- 

 sponding modifications of their mechanical proper- 

 ties. The toughness of the fibrous basis of mem- 

 branous shells imparts to them greater strength than 

 is possessed by the porcellaneous shells, which, in 

 consequence of the tenuity and uniform intermix- 

 ture of the animal cement with the calcareous 

 particles, present a harder and more transparent, 

 but at the same time more brittle compound. It is 

 these qualities, together with their smooth enamelled 

 surface, often beautifully variegated with brilliant 



* Poll has given a minute and elaborate description of the ap- 

 pearances of these fragments of membrane, when seen under the 

 microscope. See his fnlin work on the Testacea of the Two Sicilies. 



