258 PALEONTOLOGY OF ILLINOIS. 



agged crest ; their worn apices show distinct punctate structure, 

 intimately resembling the eroded coronal surfaces of Orodus and 

 other teeth occurring in these deposits, the intertubercular spaces 

 showing the ordinary striato-punctate markings of ichthyodorulites. 

 The fragment does not reveal the thickness of the walls nor any 

 part of the pulp cavity; it evidently belonged to a medium-size 

 spine. 



Compared with Oracanthus obliquus of the Keokuk limestone, the 

 present form is distinguished by its more rigid outline and the more 

 regular distribution of the tubercles. In both forms the oblique 

 transversely elongate tubercles are asymmetrical; in other words, 

 the crest culminates in a submedian apex along the angularly rounded 

 border, from which the longer lateral crest declines obliquely up- 

 ward and backward in the Keokuk spine on the right side of the 

 anterior angle, and in the present form on the left side of the less 

 angularly rounded anterior border. We are, however, inclined to 

 regard the unique example of the Keokuk species as pertaining to 

 the anterior edge of the spine, and the Chester form here alluded 

 to as possibly belonging to the opposite or posterior edge. Ex- 

 amined in the relative positions thus indicated, the direction of the 

 obliquity of the transverse rows of tubercles in the unique represen- 

 tatives of the species mentioned, merely indicate the opposed positions 

 of the respective fragments, in both of which the oblique transverse 

 disposition of the tubercles is essentially the same. If the relative 

 position of the fragment of the present spine is correctly inferred, 

 it differs from the form occurring in the St. Louis limestone, 0. 

 vetustus, Leidy, in the much less marked obliquity of the trans versly 

 elongate tubercles near the anterior border, in which latter respect, 

 however, the latter species shows variations approaching the con- 

 dition observed in the present spine. But the relations of these 

 spines can only be determined from the examination of more com- 

 plete and better preserved individuals in the case of the present 

 species and that from the Keokuk formation. 



Geological position and locality: Chester limestone ; Chester, Illinois, 



