A GROUP OF SAND-DWELLERS. 89 



There are also very many lines of growth. 

 These lines are markings, showing the shape and 

 size of the shell at all the stages of its growth. 

 You can see some of these lines in the picture. 



Figure 42. 



The last one marks the outline of the shell; then 

 as you go upward they are nearly the same shape, 

 but grow smaller and smaller, till they meet at 

 the point of the shell which is called the umbo. 



In this way the simple story of the moUusk's 

 life is written on the outside of its shell. Some- 

 times a valve gets injured while the mollusk is 

 alive. If the damage is not too great, he bravely 

 goes to work to repair it, and since he works 



