FORMATION OF SHELLS. 237 



diate textures between the membranous and the 

 porcellaneous. 



All those surfaces of the shell on its outer side 

 which are not in contact with any part of the 

 animal, are originally covered with an epider- 

 mis : * which, however, is frequently rubbed off 

 by friction. 



The process employed by nature for the for- 

 mation and enlargement of the shells of the 

 mollusca was very imperfectly understood prior 

 to the investigations of Reaumur, who may be 

 considered as having laid the first solid founda- 

 tions of the theory of this branch of comparative 

 physiology, f His experimental inquiries have 

 fully established the two following general facts : 

 first, that the growth of a shell is simply the 

 result of successive additions made to its surface ; 

 and secondly, that the materials constituting 

 each layer, so added, are furnished by the or- 

 ganized fleshy substance, which he termed the 

 skin of the animal, but which is now known by 

 the name of the mantle, and not by any vessels 

 or other kind of organization belonging to the 

 shell itself. 



If a portion of the shell of a living snail, for 

 instance, be removed, which can be done without 

 injury to the animal, since it adheres to the flesh 



* This membrane has been termed the Periostracum. 

 f Memoires de 1' Academic des Sciences, 1709, p. 367, and 

 1716, p. 303. 



