VEKTEBKATES. 245 



even depends the determination of the generic relation of the spine. 

 Specifically, however, the specimen is clearly distinct from any form 

 heretofore obtained from the same formation, and, judging from 

 such of the superficial characters as are actually discernible, the 

 spine might be provisionally identified with Acondylacanthus. 



The spine gradually tapers, and is moderately arched along the 

 anterior edge, the inclination making an angle of about 45 with 

 line of insertion, lateral surfaces gently convex transversely, and 

 very gradually converging toward the obtusely-rounded anterior edge, 

 slightly compressed to the postero-lateral angles, the posterior wall 

 broken away, exposing the oval- shaped pulp cavity lying within the 

 posterior half of the spine. The exposed lateral surfaces of the 

 spine are quite uniformly fluted longitudinally, the costse present- 

 ing comparatively narrow rounded ridges, rarely bifurcating or 

 implanted, and spaced by about their own width by the shallow 

 intercostal grooves, the anterior edge apparently occupied by a 

 wider ridge, vestiges of the usual enamel layer are discernible but 

 not accompanied by visible nodose ornamentation. The base was 

 deeply embedded in the integument, and channeled behind by the 

 continuation of the pulp cavity. The spine preserves a length of 

 14 centimetres ; the antero-posterior diameter, just above the dorsal 

 line, 2.5 centim., which, however, does not represent the actual 

 diameter on account of the imperfect condition of the posterior 

 face ; lateral diameter at same point, 9 millimetres. 



The typical forms of Acondylacanthus are more slender, and pro- 

 portionately narrower, than the above described form, and should 

 the latter prove to possess nodose costse, it should be transferred 

 to the genus Ctenacanthus. But as has been already remarked, in 

 the absence of any such ornamentation and the apparent smooth 

 plain costse, its affinities are clearly with the above genus. 



Geological position and locality: Keokuk limestone ; government 

 canal excavations at Keokuk rapids, near Keokuk, Iowa. 



