458 PALAEONTOLOGY OF ILLINOIS. 



of this marked similarity, it would seem highly improbable that the 

 spines from the two periods are of more remote relationship than sub- 

 generic, if, indeed, they prove not to be congeneric. 



The spine described by Messrs. Newberry and Worthen under 

 the name Drepanaeqnthus anceps (111. Pep., Vol. II, p. 122, PI. 12, fig. 8), 

 from the Upper Coal Measures near Springfield, Illinois, is generically 

 identical with the form above alluded to ; making with the two addi- 

 tional forms noticed below, in all four species at present known from 

 the Coal Measures. 



Xystracanthtts mirabilis, St. J. and W. 



PI. 20, Kg. 1. 



Spine large, moderately curved, anterior and posterior margins rather 

 rapidly converging towards the apex, compressed laterally ; sides flat, 

 obtusely rounded in front, abruptly defined along the obtuse postero- 

 lateral angles of the gently arched posterior face, which is rather 

 strongly keeled with slightly channeled spaces on either side, which 

 slope at an angle of about 35° ; pulp-cavity posterior to the median 

 line, probably oblong or obtusely elliptical in transverse section, lateral 

 diameter less than half the greater diameter, lateral and posterior walls 

 comparatively thin. Base unknown. Superficial tubercles variable in 

 form and size iu different parts of the spine, arranged in close longitu- 

 dinal rows, which increase by implantation towards the base, where 

 they are relatively much smaller than nearer the apex; in general 

 appearance they are slightly constricted at base, abruptly risiDg along 

 the inferior border into an obtusely angular and acuminate transverse 

 crest, above which the surface gradually descends in a broad, smooth 

 convexity to the low-rounded, inbeveled superior border, and ornamen- 

 ted by more or less distinct, irregularly disposed sharp carinse; in front 

 towards the base, the median line is occupied by a row of medium-sized, 

 narrow tubercles which extend probably two inches above the dorsal 

 line of insertion, flanked on either side in the lower half by one, two or 

 three rows of similar denticles, the exterior row continuing above in 

 larger eccentric transverse prominences which gradually change into 

 vertically elongated, high crested tubercles towards the apex, forming 

 a prominent row on either margin of the median line throughout nearly 

 the entire length of the exposed spine; behind the latter one or two 

 rows of similar but much smaller tubercles occur on either side, fol- 

 lowed by numerous rows which successively diminish iu size to the 

 postero-lateral angles, the last row being composed of slightly larger 

 denticles than the preceding ; near the base there are forty five to 

 fifty of these rows, which diminish to half that number two-thirds the 



