PLATE XV 



PAGE. 



Pig. 1. Ctenacanthus spectabilis, St. J. ami W 420 



1 a. Side view, tip and postero-inferior border partially restored in out'ine; 1 b, c, trans- 



verse sections at broken extremity and i ear basal line; 1 d, enlargement of costae 

 in the middle near base; 1 e, enlargement of costas of anterior margin above, 

 showing their appearance wheu worn. Kmderkook beds ; LeGrande, Iowa. 

 Fig. 2,3. Ctexacanthus guadocostu*, St. J. and TV 425 



2 a. Side view of fragment of spine : 2 b, enlargement of a section from the upper extrem- 



ity, showing details of ornamentation; 2 c, view of section of posterior face; 

 2 d, e, transverse section and enlargement showing contour of posterior face ; 2/, 

 enlargements in outline, showing elevation of costre from the side and end. 

 Upper Burlington ; Quincy, Illinois. 



3 a. Fragment from near the apex of a specimen referred to the above form, enlarged ; 3 b, 



enlargement of posterior edge, and section showing outline of the anterior ridge. 

 Same horizon, Louisa county, Iowa. 



Fig. 4, 5. Ctenacanthug excavatus, St. J. and TV 428 



4 a. Side view of a broken spine, enlarged about two diameters, 4 &, further enlargement 



of anterior ridge; 4 c, view of posterior face near extremity, showing position 

 of denticles and median concavity, enlarged about two diameters ; 4 d, transverse 

 section, enlarged. Keokuk limestone ; Bentonsport, Iowa. 



5 a. Side view of a fragment showing the tip of a larger spine, and transverse section of the 



same. Same horizon ; LaGrange, Missouri. 

 Fig. 6,7. Ctexacanthus Burlixgtonexsis, St. J. and TV ..426 



6 a. Side view of upper portion of spine ; 6 6, enlargement of posterior border; 6 c, trans- 



verse section. Upper Burlington ; Louisa county, Iowa. 



7 a. Side view of lower portion of spine, base restored in outline ; 7 6, enlargement of pos- 



terior margin, showing relatively smaller and obtusely conical denticles than 

 occur in the upper portion ; 7 c, enlargement along the anterior border, slrowing 

 ornamentation of three or four tuberculose costa? ; 7 d, enlargement showing the 

 anterior ridge and laterally compressed tubercles ; 7 e, section of posterior face ; 

 ~' f, 3, transverse sections, nat. size and enlarged. Same horizon and locality. 



Fig. 8. Ctenacakthtjs Kkokuk, St. J. and TV .427 



& a. Side view of large, we 1-preseived specimen, the broken inferior extremity showing 

 mould of pulp-cavity ; the ornamented costre in front partially restored; 8 6. 

 transverse section enlarged : 8 c. enlargement of posterior border, showing sculp- 

 tured denticles, bifurcated and implanted costffi, etc.; 8 d, enlargement of the 

 anterior margin, showing tuberculose cost a?, etc.; 8 e, enlargement of the ante- 

 rior ridge, showing depressed tubercles. Keokuk limestone; Boonvills, Mo. 



Fig. 9,10 Ctenacanthus gemmatus, St. J. and W 429 



9 a. Side view of imperfect spine, sin hire ornamentation partially restored; 9 6, transverse 

 section. St. Louis limestone ; Alton, Illinois. 



10 a. Enlargements from a small fragment of a spine, showing anterior ridge and several 



costs from the front, middle and posterior portions of the spine; 10 6, anterior 

 ridge, enlarged. Same horizon and locality. 

 Fig. 11. Ctenacanthus si mi lis, St. J. and TV 431 



11 a. Side view of upper extremity of spine; II b, enlargement of the anterior and two of 



the lateral costse, showing the obliquely striated lateral margins of the anterior 

 ridge, etc.; 11 c, enlargement of anterior ridge ; 11 d, transverse section. Ches- 

 ter limestone: Chester. Illinois. 



