FIRST BOOK OF ZOOLOGY. 

 BAG 



FIG. 8. ILLUSTRATING THE INCREASE IN SIZE OF A SHELL. 



Among the lot of shells collected by the pupil, different- 

 sized ones will be found.' Now, if a number of these, of 

 different sizes, can be picked out, provided they belong to the 

 same kind, or species, it will be noticed that the apex of all 

 of them will be the same ; but that the shells have increased 

 in size at the aperture, and the aperture will be larger, and 

 the larger shells will have more whorls than the smaller 

 shells. The following figures illustrate four different ages 

 of the same species of shell : 



Fro. 9. ILLUSTRATING DIFFERENT AGES OF THE SAME SHELL THE LOWER FIGURES REPRE- 

 SENTING A VIEW OF THE SHELL FROM THE APEX. 



