46G PALEONTOLOGY OF ILLINOIS. 



ing a comparatively thin plate, more or less laterally expanded poste- 

 riorly from the angular ridge in front, with more or less prominent 

 marginal angles behind. Pulp-cavity moderately largo, similar in sec- 

 tion to the body, and occupying the posterior two- thirds of the diameter 

 of the spine. The spines thus far determined are of small size. 



The present genus is founded upon Homacanthus rectus, IS. and W., 

 which constitutes the sole representative species at this time known. 

 It is perhaps more intimately allied to Amacanthus than it is with Xys- 

 tracantlius. In costation it is strikingly like the former genus, while 

 the transversely carinated tubercles bear some resemblance to the latter, 

 though their convexity is reversed, besides they are less individualized, 

 their connection with the costse being more intimate than is the case 

 with the tubercles in either Xystracanthus or Batacantlius. The uniform 

 distribution of costa? in the anterior region as well as in the posterior 

 portion, and the peculiarly expanded base, constitute its most marked 

 contrast with the former genus. Were the posterior face uniformly 

 ridged with the lateral surfaces, which it certainly appears to be, its 

 affinities should be transferred to Batacantlius, probably holding an 

 intermediate relationship with that genus and Amacanthus. 



In the anteroposterior compression, especially as observed in the 

 flattened base of iusertion, it is interesting to note the analogy to the 

 sting with which the caudal appendage of some of the Bays (Trigon, 

 etc.) is armed. 



Makracanthtjs eectus. 



PI. XXTT, Fig. 7-9. 

 Homacanthus ? rectus, Newberry and Wortben, 1866. Geol. Sur. 111., Vol. IT, p. 115, PI. XII, fig. 6. 



Dorsal spines of small size, deeply imbedded, inclined backwards at 

 an angle of about 50° with the dorsal line, very gently curved forward, 

 gradually and uniformly tapering towards the extremity, transverse 

 section subtriangular. Lateral faces gently arched, rapidly converging 

 to the rounded anterior margin, beveled along the posterior margins, 

 with five to eight moderately prominent angular costre, closely arranged 

 and rarely bifurcated in the posterior portion and the three or four 

 ridges occupying the anterior border, but interrupted in the anterior half 

 by a row of large tubercles, which often extends balf way or even more 

 towards the base in mature individuals, flanking the margin on either 

 side, and extravagantly developed near the apex, which they terminate 

 in- two strong beak-like hooks; these singular tubercles are more or less 

 laterally compressed, the thin concave inferior slope rising into the 

 more or less acutely rounded apex, the superior slope abruptly descend- 

 ing in a broad convexity, merging laterally in the keeled crest, enameled 



