STRUCTURE OF SHELLS. 171 



of late years been recognised and applied to the buildins; of 

 ships, namely, that of the diagonal arrangement of the frame- 

 work, and the oblique position of the timbers, is indentical 

 with that which from the beginning of creation, has been 

 acted upon by nature in the construction of shells. 



When the form of the crystals is prismatic, the fibres are 

 short, their direction is perpendicular to the surface, and 

 the prisms are generally hexagonal. This structure is ob- 

 servable in the Teredo giganiea from Sumatra,* and also 

 in many bivalves, such as those belonging to the genera 

 Avictda and Pinna. 



When porcellaneous shells are subjected to the solvent 

 action of acids, the animal matter in their composition offer- 

 ing but little resistance, there is a considerable and long con- 

 tinued effervescence. The solution of the carbonate of lime 

 proceeds rapidly, in consequence of the speedy disintegra- 

 tion 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. Poli has given a minute and elaborate descrip- 

 tion of the appearances of these fragments of membrane, 

 when seen under the microscope.! 



The difference between the textures of these two kinds of 

 shell is farther illustrated by the impression made upon them 

 by fire. Porcellaneous shells, when exposed to a red heat, 

 give out neither smell nor smoke; they lose, indeed, their 

 colour, but retain their figure unaltered. JNIembranous 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 corresponding modilica- 

 tions of their mechanical properties. The tougliness of the 

 fibrous basis of membranous shells, imparts to them greater 



* In this shell the crystalline appearance is so perfect, that when some 

 frag-mcnts were sent to England they were mistaken for a minei-ai production. 

 Home; Lectures, I. 5o. 



\ See hb folio work on the Testacca of the Two SiciUcs. 



