VEETEBEATES. 243 



lateral surfaces are occupied by more or less regular, prominent, 

 rounded, smooth enamel-coated costse separated by narrow intercos- 

 tal furrows, the costse comprised within the narrow compressed belt 

 along the postero-lateral margins being more slender, and toward 

 the base those occupying the body of the spine bifurcate, the ante- 

 rior ridge especially sending off frequent slender branches to the 

 dorsal line. The lower part of the spine shows distinct lines of 

 growth exactly corresponding in curvature to the dorsal line and 

 forming faint oblique annular ridges crossing the costse at irreg- 

 ular intervals ; otherwise the costse are destitute of ornamentation, 

 the occurrence of the oblique ridges mentioned not producing an 

 effect comparable with that of the tuberculated Ctenacanthi. The 

 dorsal line makes an angle of 40, and less, with the anterior edge, 

 somewhat sigmoidal in curvature, rising well up the postero-lateral 

 angles ; inserted base deep, surface irregularly striato-punctate. 

 Length of spine 12 centimetres ; greatest transverse breadth 13 mili- 

 metres. 



The above described species is represented by a unique and nearly 

 perfect specimen, which was discovered by Mr. A. S. Tiffany, to 

 whom we are indebted for the opportunity to examine many inter- 

 esting fish-remains from our Carboniferous and Devonian formations. 

 In accordance with our understanding of the values to be ascribed 

 to external characters in determining the limits of genera as repre- 

 sented by the fin-defenses of these early Selachians, the present 

 spine does not reveal any feature markedly at variance with those 

 ascribed to Acondylacanthus, with which we have identified it. 

 Specifically, however, it offers tangible and most pronounced pecu- 

 liarities, such indeed as may not readily escape notice even in 

 fragmentary material, when sufficient remains to show its peculiar 

 superficial features and the outline of its transverse section. Were 

 it possible to decide the association of teeth and fin spines, we 

 might in many instances find much less dissimilarity in the fin de- 

 fenses of different genera than that indicated by their respective 

 dentition. But lacking these facts relating to generic identity, we 

 must seek to discover such characteristics as seem to have a com- 

 mon significance and by which these varied remains may be brought 

 into such natural groups as appear most consistent with the fea- 

 tures they possess in common. 



Geological position and locality: Upper Coal Measures; upper 

 limestone at Peru, Illinois. 



