114 PALAEONTOLOGY OF ILLINOIS. 



cular carinee, separated by surface's which are striated; anterior 

 margin acutely carinated, and set with a row of small tuber- 

 cles distant from each other about the width of the spine at its 

 middle point ; lateral surfaces occupied above by 3, below by 

 5 or 6, longitudinal carinas, each tuberculated in the same man- 

 ner as the anterior edge, but the tubercles somewhat more 

 closely set; the spaces between the carinas flat, and marked 

 with wavy, longitudinal strise ; posterior border sulcated, and 

 each side of the sulcus set with closely approximated teeth. 

 At, and above the middle of the spine, these denticles are very 

 small, conical, contiguous, and scarcely depressed; toward the 

 base they become much larger, more remote and strongly hook- 

 ed downward. Entire length, 2 inches, 2 lines ; breadth at 

 base, 1 \ lines ; at base of ornamental part, 3 lines ; immersed 

 base, 4 lines. 



This species may be readily distinguished from the succeeding one by its 

 strongly arched form, its swollen base, and posterior denticles very small and 

 numerous above, larger and sparser below, while in H ? rectus they are largest 

 near the summit. From the species of Homacanthus described by Agassiz and 

 McCoy, it is clearly distinct specifically, and perhaps generically. The genus 

 Homacanthus was established by Agassiz upon some very small spines from the 

 Devonian rocks of Europe. In their general aspect these spines show a mark- 

 ed resemblance to our H. gibbosus and H rectus, but in the details of their 

 structure they are so unlike, it is doubtful if they should be classed together. 

 In the typical species of Homacanthus (^H. arcuatus, Ag.), the spine has 

 nearly the form, though about half the size, of H ? gibbosus; the base rela- 

 tively broad and irregular, the exposed posterior fluted longitudinally, bearing 

 numerous, small, depressed denticles on the posterior border. No detailed 

 description of these spines is given by Prof. Agassiz, and his figures are our 

 only guides in making the comparison of these two, represent spines with 

 smooth, longitudinal costse. In the third they seem to be interruptedly tuber- 

 cled or pectinated. In the general character given by McCoy the costse are 

 said to be few, strong and nearly smooth. In both the species now described 

 the ridges are distinctly tubercled. In each of the three spacies of Homacan- 

 thus, hitherto described, the denticles of the posterior border are nearly uniform 

 in size, and are strongly hooked downward; while in the spines under consid- 

 eration the denticles are very unequal in size, and unlike in form, the larger 



