INTRODUCTORY CHAPITER. 



13 



is absolutely necessary to protect its delicate body from 

 injury ; this shell is, in general, composed of much the 

 same substances as bone ; but the bone of a bird, or 

 quadruped, is formed by the agency of the blood, and the 

 particles of which it is composed are deposited by that 

 fluid, and again taken up and restored to the circulation, 

 a circumstance which does not take place in the sub- 

 stance of a shell. The shell is formed by the deposition 

 of layer upon layer, in the course of the growth of the 

 animal, and the ridges we perceive on many shells, 

 point out their periodical increase. 



It will be necessary, wlien describing the distinctions 

 between shells of different genera, to use several terms, 

 which will, unless properly defined, be, perhaps, unintel- 

 ligible to young people. The annexed diagrams will 

 explain the meaning of those of most frequent occur- 

 rence among the Mollusca. Fig. 1, [represents a uni- 

 valve shell ; fig. 2, another shell, of the same division, 



^DeaJa 



cut through the middle, for the purpose of showing the 

 columella, or pillar. Many shells, as, for instance, the 



