PLATE XXX— Continued. 



Spirifer Orestes, Hall and Whitfield. 

 Fig. 20. A normal adult individual. 



Chemung group. Rochford, Towa, 



Spirifeb Keokuk, Hall. 



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



Keokuk group. Keokuk, Iovxl. 

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



Keokuk gi-oup. Lizard Greek, Iovm. 

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



Keokuk group. Marion county, lovoa. 



Spirifer Leidyi, Norwood and Pratten. 

 Pigs. 25, 26. Dorsal and profile views of a normal example. 

 Chester limestone. Chester, Illinois. 



Spirifer increbescens, Hall. 



Figs. 27, 28. Dorsal and profile views of a i-ather 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. Cheater, Illinois. 



