118 PALAEONTOLOGY OF ILLINOIS. 



The mass of this spine was originally cartilaginous, covered with a thin, bony, 

 ornamented crust, which formed a solid bony tip at the summit. In the fossil 

 the cartilage having disappeared, the external shell has been crushed in and 

 its opposite internal surfaces brought in contact. Hence, as found, the spine 

 is flat, and except at the solid apex, has a thickness of scarcely more than two 

 lines. This is a well marked species of the genus, resembling more 0. abbre- 

 rfatus, Newb., from the Devonian limestone of Ohio, than any other described. 

 From this it differs, however, in its less oblique base, in its distinctly stellate 

 tubercles, and their partial linear arrangement. In 0. abbreviates the tuber- 

 cles are smooth, or nearly so, and are irregularly disseminated over the surface. 

 Length of posterior margin, 1 inch, 3 lines; of anterior border, 2 inches; base, 

 1 inch, 6 lines. 



Formation and locality: Keokuk limestone, Keokuk, Iowa. 



Genus CTENACANTHUS, Ag. 



Gen. Chak. — Fin-spine of moderate and large size, com- 

 pressed, gradually tapering, moderately arched backwards; 

 anterior face narrow, rounded ; posterior face concave, with a 

 moderate cavity, the lateral edges bordered by two rows of 

 curved denticles, inclined downwards; surface marked with 

 strong, longitudinal ridges and furrows, pectinated by trans- 

 verse scales or tubercles. The concealed base of moderate 

 size, rapidly tapering, finely striated. 



Cten acanthus angulatus, N. and W. 



PI. XII, Fig. 4, 4 a. 



Spine small, robust, compressed, posterior outline nearly 

 straight; anterior border arched backward; posterior margin 

 flattened or slightly hollowed, half the breadth of the lateral 

 surfaces; anterior margin rounded below, acute above, section 

 a triangle having sides twice as long as the base; anterior half 

 of the surface of the spine (more below, less above the middle) 

 occupied by distinct, parallel, continuous carinae, those on and 



