MOLlXJSCA. 69 



C. The Shell. Exoskeleton. 



Each of the two parts of which the shell is composed 

 is called a valve. 



1. How would you name these valves to distinguish 



one from the other ? 



2. What is the function of the hinge-ligament ? (Press 



the opened valves together.) 



3. Structure of the shell. (Dem.) 

 a. Identify in order, — 



(1) the external coating ; 



(2) the " chalky " portion ; 



(3) the pearly portion. 



h. Does the external coating cover the entire shell? 



c. Show by a sketch how the layers in the chalky 



and pearly portions are arranged. 



d. As the clam grows, where are the new layers of 



shell added ? 



4. Composition of the shell. (Examine a burnt shell.) 

 a. What is the appearance of the part that remains 



after burning ? 

 h. Has the animal matter or the mineral matter 

 been removed by burning ? 



c. Test the burnt shell by dropping it into a bot- 



tle of acid. What action takes place ? What 

 remains after the action has ceased ? 



d. What mineral will give the same action if treated 



with acid ? 



D. Outer Surface of the Shell. 



1. Lines of growth, concentric markings around the 

 umbones. 

 What causes these lines ? 

 2.- Draw the right valve. 



