PLATE XXX—Continued. 
Spirifer Orestes, Hall and Whitfield. 
Fig. 20. A normal adult individual. 
Chemung group. MocJcford, Iowa, 
Spirifer Keokuk, Hall. 
Figs. 21, 22. Views of a normal individual. 
Keokuk group. Keokuk, Iowa. 
Fig. 23. A small example, with short, rounded cai-dinal extremities. 
Keokuk group. Lizard Creek, Iowa, 
Fig. 24. A specimen with expended hinge ; showing the extreme of vai-iation in form. 
Keokuk group. Marion comity, Iowa. 
Spirifer Leidti, Norwood and Pratten. 
Figs. 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 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. 
