234 PALEONTOLOGY OF ILLINOIS. 



more frequently bifurcate, presenting less broad, flat surfaces toward 

 the base, and the pulp cavity apparently has greater extent toward 

 the extremity than is the case in C. Mayi. The posterior wall of 

 the spine is destroyed, revealing the deep furrow of the pulp cavity 

 the entire length of the fragment and which was probably closed to 

 the line of insertion. In <?. Mayi the posterior face was simply 

 channeled and bordered by the lateral flanges, which show traces of 

 the posterior denticles, also that the lateral surfaces bore longitud- 

 inal costse quite to the posterior edges. 



The unique example described was discovered by Mr. L. A. Cox, 

 of Keokuk, in whose honor the species is dedicated. 



Geological position and locality: Keokuk limestone; excavations 

 for the Government canal round the Keokuk rapids, near Montrose, 

 Iowa. 



CTENACANTHUS DEFLEXUS, St. J. and W. 



PI. XXII, Fig. 1. 



Fin spine of medium size, laterally compressed, moderately arched 

 along the anterior edge, the posterior face relatively strongly con- 

 cave, the spine uniformly expanding presenting a great breadth at 

 the dorsal line, and indicating a very recumbent position. The 

 lateral surfaces very gradually converge, with slight convexity, from 

 the postero-lateral angles forward, where they are regularly rounded 

 into the anterior edge ; the posterior face, in the somewhat abraded 

 specimen, is abruptly truncated, with indications of a low longi- 

 tudinal keel, the angular lateral edges bearing a row of minute, 

 vertically compressed denticles which are spaced by about their own 

 diameter. The costation of the lateral surfaces is quite regular, the 

 ridges regularly diminishing in size posteriorly, and simply convex, 

 separated by a narrow impressed line, increasing by bifurcation 

 which much more frequently occurs immediately along the dorsal 

 line, beyond which the attenuated basal walls project in a narrow 

 border from a point just above the inferior angle of the posterior 

 face. The anterior edge is occupied by a comparatively wide ridge, 

 which at intervals sends off lateral ridges. The entire exposed sur- 

 face is densely crowded with the costal tubercles, which form irreg- 

 ular subcoriical prominences with vertically plicated or carinated 

 sides, producing a stellate appearance, and which are even more 

 crowded than represented in the full-size illustration. Along the 

 worn anterior edge, where any remains of the costa3 are visible, 



