CHANGES OF SHELL WITH AGE. 



367 



old ones, especially such as would interfere with the continued 

 growth of the shell. The edge of the Mantle is applied against 

 their bases, and a kind of absorption of shelly matter seems to 

 take place, a notch being formed, which causes them to be easily 

 broken off. Various analogous changes are produced by a similar 

 action in other shells, the portions first formed being wholly or 

 partially removed. Sometimes the walls of the older portions 

 are thinned for the purpose of lightening the shell ; and in other 

 cases the top of the cone is altogether removed, a groove having 

 been formed around its interior, which renders it so weak as to 

 be easily broken off; in these last cases, the animal previously 

 withdraws itself from the part that is thus to be separated, and 

 throws a new partition across, by which the top of the shell 

 remains closed after the division. A shell thus deprived of its 

 apex is said to be decollated. 



910. It is not only by such removals, that the form of uni- 

 valve shells undergoes a great change. Sometimes additions are 



FIG. 560. PTEROCBRAS SCORPIO : a and 6. back and front viexvs of the young shell ; 

 c and d, the same of the adult. 



made to them, which completely alter their figure, so that two 

 individuals of different ages would be scarcely supposed at first 



cc2 



