PLATE XXX— Continued. 



SriRiFER Orestes, Hall mid Whitlickl. 



Fig. 20. A normal adult imiividual. 



Chemung' group. Rockford, lo^oa, 



Spirifer Keokuk, Hall. 



Figs. 21, 22. Views of a normal inilividual. 



Keokuk group. Keokuk, Iowa. 

 Fig. 23. A small example, with short, rounded cardinal extremities. 



Keokuk group. Lizard Creek, Iowa. 

 Fig. 24. A specimen with extended hinge ; showing the extreme of variation in form. 



Keokuk group. Marion county, loxoa. 



Spirifer Leidyi, Norwood aiul Pratteii. 

 Figs. 35, 26 Dor.sa! and profile views of a normal example. 

 Chester limestone. Chester, Illinois. 



Spirifer increbescens, Hall. 



Figfs. 27, 28. Dorsal and profile views of a rather large shell, with relatively short hinge. 

 Fig. 29. A smaller individual, with more extended hinge-line. 

 Fig. 30. Enlargement of the surface characters in this species. 

 Chester limestone. Chester, Illinois. 



