VERTEBRATES. 233 



ICHTHYODORULITES, 



GENUS CTENACANTHUS, Agassiz. 

 CTENACANTHUS COXIANUS, St. J. and W. 



PI. XXI, Fig. 1. 



Dorsal spines attain large size, somewhat rapidly tapering, later- 

 ally compressed, presenting a wedge-shaped transverse section, mod- 

 erately arched, and apparently but moderately inclined backward 

 judging from the slight obliquity of the line of insertion as shown 

 in the example before us. Anterior edge sharply rounded and occu- 

 pied by a single carina interrupted by strong transverse ridges; 

 lateral surfaces slightly convex, occupied by prominent costse, flat- 

 tened along their crests, and separated by deep, narrow sulci, in- 

 creasing by occasional bifurcation, and ornamented by irregular 

 transverse ridges, which present an abrupt declivity above and a 

 more gentle convexity below, apparently smooth. Posterior face 

 unknown. Pulp cavity apparently occupying less than half the 

 antero-posterior diameter, and situated entirely within the posterior 

 half of the spine. The specimen described attained a length of at 

 least 150 millimetres; antero-posterior diameter at line of insertion 

 probably 45 mm. ; lateral diameter at same point 14 mm. 



The above described species is recognized from a fragment of a 

 large spine, which shows a length of 90 millimetres, including part 

 of the base showing the line of insertion from which the compara- 

 tively erect position in the fish's back is inferred. Its relations are 

 most intimate with Ctenacanthus Mayi, N. and W., of the Upper 

 Burlington limestone, and the fish to which it belonged was a repre- 

 sentative species of that to which C. Mayi pertained. These two 

 spines are almost exactly of the same proportions and form. The 

 Keokuk specimen is, however, somewhat more robust, and is further 

 distinguishable by the greater width and depth of the intercostal 

 sulci and relatively narrower costse; the transverse carinse or nodes 

 along the anterior ridge are also narrower and less tumid, the costae 



