434 PALAEONTOLOGY OF ILLINOIS. 



considerably in different parts of the spine. Posterior face apparently 

 uniformly though moderately channeled, irregularly striated or punc- 

 tate, and bordered by relatively strong, prominent, obtusely rounded 

 postero-lateral angles, which bear prominent, acutely pointed, hooked 

 denticles, separated by about their own diameter in the upper portion 

 of the spine, but becoming smaller and more widely spaced below, and 

 probably extending well towards the base. Pulp cavity ovate in trans- 

 verse section, about one-third the lateral diameter of the body of the 

 spine. Base unknown; 



The collections of Messrs. Springer, Wachsmuth and Giles con- 

 tain several imperfect specimens of the form above described, all of 

 which show the upper extremity, and one large specimen, belonging to 

 the collection of Mr. Springer, indicating a spine probably four or 

 five inches in length when entire, exhibits the smooth, enameled costaa 

 in a perfect state of preservation. It is apparently referable to the 

 same generic group as the spines which we have described from the 

 Keokuk formation under the name A. cequicostatus. It differs from the 

 Keokuk form in the more irregularly developed costse, proportionately 

 stronger, and more numerous denticles along the postero-lateral angles, 

 less deeply excavated posterior face, and more strongly curved outline. 



Position ani locality : Not uncommon in the lower fish-bed, and appa- 

 rently the same form occurs in the upper fish-bed, of the Kinderhook 

 formation; Burlington, Iowa. 



ACONDYLACANTHUS ^QTJICOSTATTJS, St. J. and W. 

 PI. XVT, Fig. 12, 13. 



Fin-spine long, gradually tapering to the apex, and gently curved, 

 transverse section lenticular, truncated behind. Lateral faces flattened 

 or faintly convex in the posterior half, more rapidly converging to the 

 anterior margin, which is occupied by a prominent, sharply rounded 

 ridge, similar to the costse covering the body of the spine, of which 

 latter there are ten to sixteen, more or less, in either face, quite uniform 

 in size, rarely bifurcating descending, and closely arranged, the inter- 

 costal spaces narrow, or generally less than half the diameter of the 

 ridges, except sometimes along the anterior border the occurrence of a 

 broader groove above the bifurcation of the anterior ridge. Posterior 

 face deeply excavated, postero-lateral angles prominent and sharply 

 rounded, bearing along their inner edges a row of depressed, hooked 

 denticles, spaced by about or little more than their own diameter, and 

 apparently extending at least half the distance from the apex towards 

 the base. Internal cavity subelliptical in section, about half the lateral 



