4G4 PALJEONTOLOGY OF ILLINOIS. 



condition of the specimen figured by McCoy, Brit. Pal. Foss. p. G2S, 

 PL 3 k, fig. 26, 27, is attributable to compression, we have no means of 

 determining. 



The present species is dedicated to Professor McCoy, who first recog- 

 nized the generic peculiarities of this remarkable genus of ichthyodoru- 

 lites, as a slight testimonial of appreciation of his labors in this field 

 of paleontology. 



Position and locality: Not uncommon in the upper beds of the St. 

 Louis limestone; Alton, Illinois, and St. Louis, Missouri. 



Genus AMACANTHTTS, St. J. and W. 



Dorsal spine firmly implanted, curved forward, more or less laterally 

 compressed, posterior face truncated and longitudinally keeled or den- 

 ticulate along the median line, rounded in front along the concave ante- 

 rior margin, which is occupied by a row of more or less strongly developed 

 tubercles, which extend from apex to base, lateral surface longitudinally 

 traversed by irregularly spaced, bifurcated, tuberculose costa?, that bor- 

 dering the postero-lateral angles bearing laterally compressed denticles 

 more or less similar to the posterior keel, the tubercles in the front rows 

 radiately sculptured. Spines of small or medium size. 



In the above genus is recognized the form originally described by 

 Messrs. Newberry and Worthen under the name Homacantlius gibbo- 

 sus. As inferred from the above generic diagnosis, the spines here 

 referred to are entirely distinct from Homacanthus, Agassiz, their affini- 

 ties being with Xystracanthus, from which latter they are distinguished 

 by the peculiar truncated or serrated condition of the posterior keel, as 

 also the character of the costation and the tuberculose anterior region. 



Amacanthus gibbosus. 



PI. XXH, Fig. 6. 



JBomacanthus gibbosus, Newbkrby and TVoethen, 1866, Geological Survey of Illinois, Vol. II, p. 113, 



PI. XII, Tig. 1. 



The species is characterized by the gentle, regular curvature of the 

 spine, which gradually tapers from the somewhat expanded base to the 

 obtusely pointed apex, laterally compressed, subelliptical in transverse 

 section, anterior margin angularly rounded, supporting numerous, rela- 

 tively strong, acute, generally slightly reeurved, vertically sculptured 

 tubercles, which rise from a broad base, closely and regularly spaced 

 in the upper half or two-thirds, gradually increasing in size towards the 

 base, where they are apparently less regularly and more widely spaced. 

 The lateral faces present five to seven, or more, relatively strong costae, 



