12 Ctenacanthus Spines from the Keokuk Limestone. 



apart, and decreasing in size toward the posterior face. Each of 

 the costse is denticulated by numerous sharp folds, which extend 

 half way across the intercostal spaces, and are separated from 

 each other by about the thickness of the ridge. The anterior 

 edge of the spine is formed by a ridge wider than the rest, and of 

 compound origin. 



The second icbthyodorulite that deserves notice represents a 

 new species of Ctenacanthus. It will be seen from the 

 annexed illustration that the spine is very nearly 

 perfect, only the extreme tip being broken away. 

 It is 12*5 cm long, and not quite l*5 cm in maximum 

 width. Its stylet-shaped form, nearly rectilinear 

 edges, and the nature of its ornamentation readily 

 distinguish it from all other species. In cross-sec- 

 tion it is less compressed than most spines belong- 

 ing to Ctenacanthus. The posterior face is embed- 

 ded in the matrix, but the latter has been removed 

 along one edge sufficiently to reveal a series of 

 small denticles standing at right angles to the axis 

 of the spine. A convenient fracture makes it pos- 

 sible to remove the base, thus displaying the pulp- 

 cavity at that point. It is of ample size, and is 

 continued for a short distance upward in an open 

 groove as in other species. Another fracture 

 higher up presents the cross section shown in fig. 

 2i?,. from which it will be seen that a posterior keel 

 is present. 



Owing to abrasion, the longitudinal costse appear 

 nearly smooth except along the posterior edges, 

 where they retain their tuberculation. The tuber- 

 cles are quite small, and resemble those of C. keokuk / 

 a few are also distributed in the intercostal grooves, 

 thus indicating an approach to Asteroptychius. 

 Besides bearing tubercles, the costse appear to have 

 been denticulated along their sides, as indicated by 

 transverse markings in many of the intercostal 

 spaces. The ridges are triangular in cross-section, 

 increase in number downward by dichotomy, and 

 are much crowded towards the posterior face ; some 

 of them appear longitudinally striated when worn. 

 The anterior margin of the spine is formed by the 

 concrescence of several ridges, and becomes thickened in con- 

 sequence. 



The definition of this species may be summarized briefly as 

 follows : 



a 



ffJJOS 



O 



m 



