2 26 AN INTRODUCTION TO THE STUDY OF FOSSILS 



adductor impression absent, posterior nearly central. No 

 siphons present. 



The shell becomes attached at the close of its free-swimming 

 stage as soon as it settles to the bottom of the water. The 

 mantle at the front (ventral) edge of the left valve when coming 

 in contact with some foreign object secretes a cement similar 

 to the organic material within the shell itself. 



Oyster shells are very often found to be completely riddled 

 with small holes, the work of the boring sponge, CHona sul- 

 phur ea. 



Fossil oysters are found in North America, Europe, and 

 India. They are especially abundant in the Cretaceous of 

 America and in the Tertiary of the coast states. A brackish 

 water form, living in tropical and temperate seas, the oyster 

 thrives best in clear water close to the shore where the water is 

 fresher than in the open sea. 



1. Sketch interior view of one valve, noting the characteristic 

 V-shaped ligament area and the muscle marking. 



2. Explain the action of the ligament in opening or closing 

 the shell. 



3. How does Ostrea differ from Nucula in {a) number of muscle 

 impressions; {b) teeth? 



4. Discuss the reason for the central location of the posterior 

 adductor impression. 



5. Is Ostrea attached or free during life ? How can this be 

 told from the shell ? 



6. In what kind of water does it usually live ? 



7. The oyster is a good example of adaptation to a sedentary 

 life. How does this show in the shell ? 



8. Notice the difference in the shape and size of the valves. 

 Why is this desirable ? 



9. Would such a heavy shell be satisfactory for the scallop 

 (Pecten)? Why? 



10. Sketch longitudinal vertical section of a valve, showing 

 the progression of the muscle impression and thickening of the 

 valve. 



11. Why does Cliona bore into such shells as the oyster ? 



