372 PALAEONTOLOGY OF ILLINOIS. 



LISTRACANTHUS HYSTRix, N. and W. 



PI. ii, fig. 3, 3a. 



SPINES small, delicate, thin, flattened, broad below, rapidly 

 narrowed above, gently arched in outline, both edges set with 

 sharp, spiny teeth directed upward; the sides marked with 

 fine longitudinal ridges, which successively terminate above 

 in the margin. 



Two specimens of this distinctly new form are before us, of which one is 

 three inches, the other, one inch in length. In the larger specimen, the mar- 

 ginal teeth are very numerous, of unequal size those of the concave margin 

 being much the larger. In the smaller specimen they are fewer, relatively 

 larger, and about equal on both margins. The base is the same in each, ob- 

 liquely truncated, and expanded like a trumpet mouth, indicating that it was 

 set on the surface of body or head, and had not been inserted in the integu- 

 ments. 



Formation cind locality : Coal Measures ; Vermilion county, 111. 



GENUS CTENACANTHUS, Ag. 



CTENACANTHUS MAYI, N. and W. 



PL ii, fig. 2, 2a. 



SPINE of medium size, six to eight inches long by one and 

 a-quarter broad at base, much compressed, gently arched back- 

 ward ; summit sub-acute, smooth ; anterior and convex mar- 

 gin sub-acute and marked by relatively large and remote an- 

 nular tubercles; posterior margin furrowed longitudinally and 

 striated transversely, the salient edges set above with remote 

 and small denticles ; sides flattened, and ornamented by ten 

 or more strong longitudinal costse bearing flattened annular 

 tubercles. 



This is a typical species of the genus, and perhaps the most beautiful yet 

 discovered. Its outlines are regular and elegant; the ornamentations very 

 elaborate and distinct. It may be readily distinguished from all other known 



