STRUCTURE OF SHELLS. 235 



building of ships, namely, that of the diagonal 

 arrangement of the frame- work, and the oblique 

 position of the timbers, is identical 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 perpen- 

 dicular to the surface, and the prisms are gene- 

 rally hexagonal. This structure is observable 

 in the Teredo gigantea from Sumatra,* and also 

 in many bivalves, such as those belonging to the 

 genera Avicula and Pinna. 



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 effer- 

 vescence. 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. Poli has given a minute and elaborate 

 description of the appearances of these fragments 

 of membrane, when seen under the microscope.')' 



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

 when some fragments were sent to England, they were mistaken 

 for a mineral production. Home ; Lectures, I. 53. 



f See his folio work on the Testacea of the Two Sicilies. 



