FIRST BOOK OF ZOOLOGY. 



Fia. 8. — Illttsteating the Increase rs Size of a Shell. 



Among tlie lot of shells collected by tlie 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 : 



Fig. 9.— iLLUBTRATrao Different Ages or the same Shell— the Lotvee Figures repre- 

 senting A View of the Shell from the Apex. 



