PLATE XXI. 



PAGE. 



CTENACANTHUS COXANUS, St. J. and W 233 



Fig. 1 a. Side view of spine, outline restored; 1 b, do., showing anterior edge; 1 c, do., 

 posterior face, the wall broken away exposing the pulp cavity; 1 d, do., 

 transverse section near middle. Keokuk; Montrose, Iowa. Mr. Cox. 



CTENACANTHUS PELLENSIS 237 



Fig. 2 a. Fragment of spine, showing the dorsal or anterior edge; 2 b. do., side view, 

 outline partially restored; 2 c, do., posterior face, showing beveled lat- 

 eral edges and the deep trough of the pulp cavity; 2 d, do., enlargement 

 of surface, showing variable character of superficial ornamentation; 2 e 

 do., cross section. St. Louis formation; Pella, Iowa. Mr. Van Home. 



CTENACANTHUS CANNALIRATUS, St. J. and W 239 



Fig. 3 a. Side view of fragment of spine; 3 b, do., anterior or dorsal edge; 3 c, do., pos- 

 terior face; 3 d, do., transverse section. Chester; Chester, 111. Dr. Ham- 

 bach. 



ASTEROPTYCHIUS TENELLUS, St. J. and W 248 



Fig. 4 a. Fragment of spine, view of left side: 4 b, do., enlargement of surface; 4 c, do., 

 transverse section. Upper. Coal Measures; Topeka. Mr. St. John. 



