242 PALAEONTOLOGY OF ILLINOIS. 



in front bearing two rows of minute punctures and a faint median 

 filiform line, the costse enveloped in a smooth coating of enamel 

 without transverse or tubercular elevation. Near the middle of the 

 exposed right side of the type specimen, one of the costse attains 

 double the breadth of those in front, and immediately along the 

 beveled posterior edges occurs a plain space apparently destitute of 

 longitudinal ridges. Pulp cavity compressed ovate in outline, con- 

 fined within the posterior half, and inclosed between thick walls. 

 A specimen preserving a length of 65 millimetres has a breadth at 

 base of 6.5 mm., and a thickness of 3 mm. 



We have met with only a single example of the above described 

 ichthyodorulite, which presents the greater portion of a small spine 

 extending from the acute distal extremity to a point near the dorsal 

 line, above which it is broken off. The specimen is embedded in a 

 limestone matrix in such manner as to allow of the exposure of the 

 right side and the posterior face; it is slightly distorted and frac- 

 tured across at several places, revealing the small pulp-cavity, and 

 somewhat worn especially along the posterior half of the lateral 

 surfaces. The anterior costse, however, are uninjured, and their 

 condition is as noted above. The striking characteristics of the 

 spine are its slight curvature, rigid outline, and the deeply channeled 

 posterior face and closely set large denticles. The specimen has no 

 near ally in the Coal Measure strata, and so far as it is possible to 

 determine to the -contrary, it presents all the characteristics ascrib- 

 able to Acondylacanthus. 



Geological position and locality: Upper Coal Measures, above coal 

 No. 9, 111. Gen'l Sec. ; the upper limestone at La Salle, Illinois. 



ACONDYLACANTHUS NUPERUS, St. J. and W. 



PI. XXVI, Fig. 3. 



Dorsal spine of medium size, rather strongly arched along the 

 anterior edge and somewhat rapidly tapering, laterally compressed, 

 transverse section cuneate ; posterior face narrow, deeply channeled ; 

 pulp cavity relatively small, sublenticular or compressed ovoid in 

 transverse outline. Lateral surfaces moderately convex transversely ; 

 sharply rounded into the anterior keel, also gently rounded and 

 compressed posteriorly to the postero-lateral angles, which bear a 

 row of laterally compressed, strongly downward-hooked denticles en- 

 circled at the base by a cincture, more or less regularly spaced by 

 less than their own greater diameter, and extending two-thirds or 

 more the length of the exposed portion of the posterior face. The 



