446 PALAEONTOLOGY OF ILLINOIS. 



to the generic identity of these two individuals, and their specific rela. 

 tionship is doubtless most iutimate. However, the form under consider- 

 ation appears to be especially distinguished, as compared with that 

 cited above, by its relatively broader base, the striation of which is pos- 

 teriorly deflected instead of being nearly parallel with the anterior edge 

 as represented in the Tennessee specimen, the tubercles are relatively 

 smaller and more numerous, with distinct cutting edges, and more finely 

 striated vertically, their apices being directed upward, though trunca- 

 ted from wear, as mentioned in the description of B. siderius. 



Position and locality : In the upper beds of the St. Louis formation ; 

 Alton, Illinois. 



Genus GLTMM AT ACANTHUS, St. J. and W. 



Fin-ray apparently vertically elongated and posteriorly arched, late- 

 rally compressed, the lateral faces converging to the more or less sharply 

 rounded anterior margin, and occupied by stellate or vertically striated 

 tubercles, arranged in longitudinal rows more or less nearly parallel 

 with the anterior edge, but increasing by implantation below, and often 

 disposed in irregular transverse or oblique rows. Posterior face and 

 base unknown. Pulp cavity posterior occupying half, more or less, of 

 the transverse diameter of the spine. 



The solitary fragment of ichthyodorulite affording the above diagnosis, 

 would doubtless be regarded as probably referable to the Mesozoio genus 

 recognized by Prof. Ag-assiz under the name Asteracanthus, but we 

 believe we are justified in recognizing the distinction between them as 

 based upon even so indifferently preserved specimen as the example 

 before us. The tubercles are much more closely arranged laterally, the 

 interspaces between the longitudinal rows beiug exceedingly narrow 

 aud scarcely at all depressed; the tubercles are also much more deli- 

 cately sculptured and their stellate character far less conspicuous than 

 is the case with the AsteracantM. 



In the disposition and form of the tubercles there exists marked 

 resemblance to the BrepanacantM ; but the fragment here noticed pre- 

 sents in the apparent, however slight, posterior deflection in the descent 

 of the rows of tubercles evidence of the posterior curvature of the 

 spine, which fact precludes its reference to the latter genus. 



The tendency of the tubercles to assume more or less irregular trans- 

 verse or oblique disposition, also recalls a similar arrangement of the 

 tuberculatum in the Oracanthi ; the massive structure and symmetrical 

 proportions, however, together with the probable truncation of the pos- 

 terior border, serve to define it from the typical examples of that genus. 



