FRESH-WATER SHELLS. 5 



5. If the surface of the shell be examined closely, deli- 

 cate lines running from one suture to another will be seen, 

 as in the figures already given ; and, if the shell be looked at 

 from the side of the aperture, these lines will be found run- 

 ning parallel to the edge of the aperture, or lip, as it is 

 called. 



These delicate lines are called lines of growth. 



FIG. 7. SHOWING LINES OF GROWTH RUNNING PARALLEL TO THE EDGE OF THE APERTURE. 



The shell is increased in size by successive layers of 

 shelly matter added to the borders of the aperture. In this 

 way the shell grows. 



A clearer idea of the growth of a shell may be obtained 

 by studying the next figure (Fig. 8) : A representing in out- 

 line a young shell ; B representing the full-grown shell in 

 outline ; and C representing the same outline as J?, with a 

 number of lines of growth represented upon it. 



If the shell were now to continue its growth a single half- 

 turn, or whorl, the dotted lines would indicate the increased 

 stages it would assume : a representing the first increase in 

 size, J) the next stage, and c the appearance of the shell when 

 the additional half-whorl has been completed. 



