450 PALEONTOLOGY OF ILLINOIS. 



arranged along the costae or separated by a space equal to or exceeding 

 their own diameter, very gradually diminishing in size posteriorly, but 

 presenting a marked contrast in size to the large, widely spaced tuber- 

 cles along the anterior margin, which, however, they intimately resem- 

 ble in form and radiate-sculpturing ; the iutercostal spaces about equal 

 the diameter of the ribs except along the anterior edge where they form 

 a rather wide, gently depressed area on either side. The lateral faces 

 slightly converge posteriorly, giving to the transverse section a long- 

 ovate figure, the narrow extremity of which is truncated by the poste- 

 rior face, which latter is but gently raised along the median keel and 

 faintly channeled either side, the postero-lateral edges forming an 

 obtuse angle without any sign of denticles, though such, may have 

 existed. The basal portion, as usual, is deeply excavated by the pulp 

 cavity, which latter has a clavate section in the upper partof the spine; 

 antero-inferior shoulder prominent, rounded above and moderately 

 expanded laterally, beneath which in front the base is deeply notched 

 and laterally compressed, and continued anteriorly into an obtuse point. 

 Most of the examples of this form, of -which we have had for examina- 

 tion those of less than an inch in length to the fine original of the spe- 

 cies which probably attained the length of ten inches, are in a perfectly 

 nude condition and much modified in outline by attrition. As com- 

 pared with Ph. arcuatus, McCoy, these spines are proportionately longer 

 and narrower, and probably less strongly arched forward, and, as 

 remarked by Messrs. Ne\vberky and Woethen, the tubercles are less 

 symmetrical or more eccentrically apiculate, more finely sculptured, and 

 the large anterior ones apparently isolated by wide vertical spaces. 



The chief distinction of the genus as here defined, compared with 

 Xystracanthus, Leidy, of the Coal Measures," and Drepanacanthus, N. and 

 W., consists in the relatively greater prominence of the antero-inferior 

 shoulder, and the prevalence of a plain space of greater or less breadth 

 either side of the anterior margin which is occupied by the compara- 

 tively extravagantly developed tubercles, distinctions which are proba- 

 bly of generic value. The first authentic examples of the latter genus 

 appear in the Keokuk formation, and which are very closely related to, 

 if jndeed not identical with Xystracanthus, Leidy. 



Professor Agassiz described in the Poissons Fossiles (III, PL 1, fig. 

 1 and 8,) a species of dorsal spine from the Carboniferous limestone 

 of Bristol, England, under the name Onclms hamatus, which presents a 

 striking resemblance to denuded specimens of the present genus. Sub- 

 sequently Professor Agassiz recognized the above species as the type 

 of a distinct genus, to which he gave the name Cladacanthus, Pois. Foss., 

 Ill, appended list of new forms. In a list of the species of Carbonife- 

 rous fishes contained in his collection, kindly communicated by Lord 



