232 AN INTRODUCTION TO THE STUDY OF FOSSILS 



from the Pennsylvanian. The resemblances are so numerous 

 and so close as strongly to suggest a true descent. 



The genus is of world-wide distribution. Three of the more 

 common Atlantic coast American species used as food are, 

 — P. ?nagellanicus , New Brunswick to North Carolina, P. 

 gibbus borealis of the New England coast, and P. gibbus irra- 

 dians, which ranges from New Jersey to Texas. Other species 

 are much esteemed in Europe. Only the adductor muscle of 

 scallops is eaten. Fossil forms are especially abundant in the 

 Comanchean of Texas and the Cretaceous of New Jersey. 



1. When at rest Pecten lies upon its right valve. How does 

 the right valve differ from the left ? 



2. Sketch right valve, both exterior and interior, indicating 

 ears (anterior and posterior), umbo, ribs, growth lines, paUial 

 line, byssal notch, muscle scar. Only one adductor muscle is 

 present ; is this the anterior or posterior ? 



3. Why is the byssal notch present ? 



4. Indicate with a heavier line upon your sketch the size 

 of the shell at the end of one year. Why is this growth line so 

 conspicuous ? 



5. Why are siphons, and hence a pallial sinus, not necessary 

 in Pecten ? 



6. How can you tell which is the right valve ? 



7. Mention two differences between Pecten and Venus. 



8. How does it move from one place to another ? 



9. What is the common name of recent Pectens ? 



10. How does the shell usually give indication of the age of 

 the animal ? 



11. How does the living Pecten breathe ? 



12. What portion is eaten by man ? 



13. What need has it of so large an adductor muscle ? 



14. Where are the eyes? Why? Can most pelecypods 

 see? 



15. Give briefly the development of the shell from the early 

 prodissoconch stage to the adult. 



16. Parallel this development with fossil forms. 



17. How are the plications formed? 



18. Distinguish Pecten from Pteria. 



