PLATE XVII 



PAGB. 



~Fig. 1- - Bythi acanthus Van Hornei, St. J. aud W 445 



1 a. Side view of fin-spine: 1 &, anterior aspect, reduced to one-fourth natural size; 1 c, 



view showing posterior face ; 1 d, e, /, views of tubercles showing inferior side, 

 lateral view, aDd seen from above ; 1 a, outline of an elongated tubercle, seen from 

 above ; 1 7;, transverse section near the middle of exposed portion of spine ; 1 i, simi- 

 lar section across the base midway between the anterior shoulder and the inferior 

 extremity. St. Louis limestone ; Alton, Illinois. 



Fig. 2. Glymaiatacanthus Irtsitu, St. J. and W 447 



2 a. Side view of fragment of spine, base partially restored in outline j 2 &, transverse sec- 



tion, partially restored in outline along the posterior side; 2 c, enlargement of 

 tubercles as seen from above, showing also the interspaces; 2 d, side view of a 

 single tubercle, enlarged. Kinderhook beds ; Marshall county, Iowa. 

 Pig. 3,4. Geisacanthus bullatus, St. J. andW 441 



3 a. Side view of a spine showing the base and part of the exposed portion, enlarged two 



diameters, the upper portion of the figure representing the surface denuded of 

 tubercles, as seen in fig. 4 a ; 3 b, enlargement showing one of the tubercles seen 

 from above, and lateral view in outline. Chester limestone; Chester, 111. 



4 a. Side view of an imperfect spine, showing patch of surface near tip denuded of tuber- 



cles, and below the pulp-cavity is exposed by the breaking away of^the lateral wall. 

 Same horizon and locality. 



