110 PALAEONTOLOGY OF ILLINOIS. 



Genus LEPTACANTHUS, Ag. 



Gen. Char. — Fin-spine very long, narrow, gently arched 

 backwards, very gradually tapering, greatly compressed, sides 

 flattened; anterior face thin, keeled; surface covered with very 

 numerous, longitudinal ridges and striae; posterior side set with 

 two rows of numerous, close, small, downward-curved denticles. 



LEPT ACANTHUS? OCCIDENTALIS, N. and W. 

 PI. XII, Fig. 2. 



Spine long, slender, gently arched backward, very much 

 compressed throughout, sides flat; anterior edge sharp, posterior 

 edge conspicuously furrowed and bordered by two rows of small, 

 acute, compressed and depressed hooks, of which the bases are 

 nearly in contact; lateral surfaces occupied by numerous sub- 

 equal, parallel, continuous, smooth, flattened costee, near the 

 base about 15 on either side, at the middle 9-10, and near the 

 summit 4-5, the number being increased below by the bifurca- 

 tion of those occupying the upper part of the spine. The 

 specimens before us are neither of them complete, but they 

 indicate a total length of about 6 inches, with a breadth at base 

 of about 6 lines, the average thickness being one-fourth the 

 breadth. 



We have referred these spines to Leptacantlms with much doubt, as there 

 seems to be little probability that they are generically identical with those from 

 the Oolite, which Agassiz first designated by that name. It is true the general 

 form is similar, but the striated or obscure and confusedly costate surface of 

 the typical Leptacanthi must have given them an aspect widely different from 

 that of these spines, so uniformly and regularly ribbed throughout. The sus- 

 picion that these differences are generic is also strengthened by the fact that 

 up to the present time no teeth have been found, both in the Lower Carbonif- 

 erous and Jurassic rocks, which have been referred to the same genus ; and it 

 is scarcely probably that while the fin-rays of these ancient placoids have been 

 met with in considerable numbers, that the much more numerous teeth could 



