PLATE XI Y. 



PAGE. 



pig. l. Ctenacanthus sculptus, St. J. and"W. 421 



1 a. Side view of a nearly perfect specimen; 1 b, transverse section of spine. near npper 



extremity, showing pulp cavity and contour of posterior face; 1 c, enlargement 

 showing disposition and form of tubercles, the eccentric anterior ridge near the 

 base, with profile outline; 1 d, enlargement, of tubercle of third costa from front, 

 near base, and profile outline ; 1 e,f, enlargement of tubercles from middle costee ; 



1 g, enlargement of cost® and tubercles in middle and posterior portions above. 

 Lower fish-bed Kinderhook ; Burlington, Iowa- 

 Fig. 2. Ctenacanthus varians, St. J. andW 422 



2 a. Side view of broken spine, partially restored in outline, the inner dotted lines showing 



the outline of the pulp-cavity; 2 b, view of upper half of posterior face, showing 

 the strong, rounded, median keel and denticles along the postero-lateral angles; 



2 c, transverse section about one-fouith the distance from the apex; 2 d t transverse 

 section near line of insertion, showing the deeply excavated posterior border; 2 e, 

 enlargement of costee near base in front; 2 /, enlargement of middle costee of right 

 side near base ; 2 g, enlargement of tenth costa from front in lower third of spine 

 of same-side; 2 h. enlargement of posterior costa near base; 2£, profile enlargement 

 of posterior denticles. Upper fish-bed Kinderhook; Burlington, Iowa. 



Fig*. 3,4. Ctenacanthus speciosus, St. J. and W 424 



3 a. Side view of large imperfect spine partially restored in outline; 3 6, transverse section 



below the opening of internal cavity ; 3 c, transverse section near the upper extrem- 

 ity, posterior outline restored ; 3 d, enlargement of cost® from various parts of the 

 spine; 3 e, enlargement of fragment of another individual, showing the posterior 

 denticles, etc. Lower fish-bed Kinderhook; Burlington, Iowa. 



4 a- Side view of a worn specimen, probably referable to this form, in which the decussa* 



tions of the costee are obsolete ; 4 b, view of the posterior face ; 4 c,d, transverse 

 sections near base and upper part Same horizon and locality, 



