FEESH-WATEE MUSSEL 191 



becomes flexible. What is its use ? Why are the 

 shells of dead mussels and clams always open ? 



4. The lines of growth.. — What is their position? 



Shape ? Number ? Where do they begin '{ Is 

 their number the same for the two valves of 

 the same shell? Are they everywhere at a 

 uniform distance apart ? 



5. The umbo or "beak." — What is its position? 



How is it formed ? How does it compare in 

 age with the rest of the shell? Does it differ 

 in color from the other regions ? If so, explain. 

 On the inside find 



6. The hinge-teeth (not present in Atiodonta). — 



What is their position? Shape? Structure? 

 Number on each valve ? Note the two kinds — 

 the one pointed, the cardinal teeth ; the other 

 elongated, the lateral teeth. How many of 

 each kind are there on each valve ? What is 

 their use ? 



7. The lining of the valve. — What is its color? 



How does it differ from the outside of the 

 valve ? Are there any variations in color ? If 

 so, to what are these due ? 



8. The pallial impression. — What is its position? 



How distinguished? What is its relation to 

 the impressions of the adductor muscles ? 



9. The impressions of the adductor muscles. — 



How are they distinguished ? What is the posi- 

 tion of each ? Color, shape, and size of each ? 

 Note also the paths of shifting of these muscles. 

 In front of the posterior adductor impression 

 find 

 10. The scar of the posterior retractor muscle. 

 — Compare with preceding. 



