5. 



a) 





b) 



6. 



a) 





b) 



7. 



a) 





b) 



8. 



a) 





b) 



9. 



a) 





b) 



10. 



a) 





b) 



11. 



a) 





b) 



12. 



a) 





b) 



13. 



a) 





b) 



14. 



a) 





b) 



15. 



a) 





b) 



16. 



a) 





b) 



17. 



a) 





b) 



18. 



a) 





b) 



19. 



a) 





b) 





c) 



20. 



a) 





b) 



21. 



a) 





b) 



PENNSYLVANIAN FOSSILS 113 



Shell higher than long, beaks at one end of the hinge Myalina 



Shell longer than high, beaks near but not quite at the 



end of the hinge Naiadites 



Hinge line straight 7 



Hinge line V-shaped, the wings prolonged over the beak Entolium 



Wings decidedly unequal in size 8 



Wings almost equal in size 9 



Length much greater than the height Pteria 



Height greater than the length Aviculopecten 



Outline of one wing concave (fig. 317) 10 



Outline of both wings straight or convex 11 



Radial ridges with wart-like bumps Euchondria 



Radial ridges smooth or nearly so Deltopecten 



Radial ridges grouped in pairs Lima 



Radial ridges almost equally spaced Acanthopecten 



Shell longer than high 14 



Shell higher than long or height and length equal 13 



Surface with widely spaced concentric ridges Astartella 



Surface with small, crowded concentric ridges Nucula 



Beaks nearly central 15 



Beaks nearer one end of the shell than the center 17 



Surface with widely spaced concentric ridges Edmondia 



Surface without widely spaced concentric ridges 16 



Shell with a distinct ridge from beak to margin Schizodus 



Shell without such a ridge Nucula 



Length 4 times the height Solenomorpha 



Length less 4 times the height 18 



Shell with one or more distinct ridges from beak to margin 19 



Shell without such a ridge 20 



One radiating ridge only from beak to margin Parallelodon 



Three radiating ridges from beak to margin Pleurophorella 



Four to six radiating ridges from beak to margin Pleurophorus 



One end of shell ending in a sharp point Nuculana 



Both ends of shell gently rounded 21 



Beaks at one end of the hinge Allorisma 



Beaks near, but not at one end of the hinge Cypricardinia 



