432 PALEONTOLOGY OF ILLINOIS. 



these spines may prove to be specifically identical with C. annularis, 

 representing certain conditions of preservation not apparent in tbe soli- 

 tary example examined by Messrs. Newberry and Worthen. 



Position and locality : In the lower fish-bed of the Chester lime- 

 stone ; Chester, Illinois. 



Genus ACOKDYLACANTHUS, St. J. and W. 



Fin-rays long, gradually tapering, laterally compressed, and moder- 

 ately curved posteriorly. Lateral faces longitudinally fluted, the costae 

 being smooth and enameled, increasing by occasional bifurcation, per- 

 haps more rarely by implantation. Posterior face uniformly excavated 

 longitudinally, apparently without median keel ; postero lateral angles 

 bearing a row of downward hooked denticles, which extend in the 

 majority of species well towards the base. Pulp cavity occupying the 

 posterior half or more of the spine. Base unknown. 



Regarding the generic identity of the Carboniferous ichthyodorulites 

 heretofore referred to Leptacanthus, Agassiz, expressions of doubt have 

 from time to time been made by investigators who have had to deal 

 with these remains. The genus Leptacanthus was based upon spines 

 occurring in the Jurassic formations of Europe, the oldest representa- 

 tive, as originally determined by Professr Agassiz, belonging to the 

 Lias. Subsequently Professor Agassiz identified certain Carboniferous 

 fin-spines with the genus, and other authors, at a later date, have 

 extended the list of species of the same period, however,- in every case 

 pointing out the difficulties in the way of determining the real affinities 

 of these earlier forms, and the necessity of their provisional reference 

 to the Mesozoic genus. There is one exception, that of Col. Roman- 

 owskt, who, in an interesting paper on the fossil-fishes of the Lower 

 Carboniferous deposits of Russia, describes a fin-spine which is appa- 

 rently generically identical with the Carboniferous so-called Leptqcanthi, 

 and which he has named Cladodus tenuistriatus, inferring their generic 

 identity with Cladodus from the circumstance that the spines are asso- 

 ciated with the remains of the dentition of that geuus. The ichthyo- 

 dorulites for which Professor Agassiz proposed the generic term 

 Cladacanthus in the lists of new forms appended to the Poissons Fossiles, 

 III, are cited from the British mouutain limestone, but of which we 

 have failed to gain any definite knowledge. I am aware, however, that 

 Professor Agassiz was strongly inclined to regard the spines to which 

 he gave the name Ctenacantlius as probably belonging to the fishes 

 which bore the teeth called Cladodus. The latter inference was prob- 

 ably based upon as extensive data as was the case in that of the Russian 



