VEETEBEATES. 471 



they are strongly ridged vertically, though the two largest anterior 

 rows are generally quite smooth, probably in consequence of wear. Base 

 unknown. Pulp-cavity of moderate size, in section similar to that of 

 the body, rapidly diminishing in diameter above, the posterior wall 

 exceeding the thickness of the lateral walls and about half that of the 

 anterior wall. The largest examples attain seven inches or more in 

 length. 



The species originally described by Messrs. Xewbekry and Worthen 

 was based upon an imperfect, worn specimen, obtained from the geode 

 beds immediately overlaying the Keokuk limestone. Subsequently 

 additional and in some respects more perfect material lias been obtained, 

 the study of which has afforded a clearer knowledge of the generic as 

 well as specific peculiarities by which these spines are distinguished. 



Our specimens do not show the actual extent of the denticles in the 

 anterior border, indeed they would seem not to reach beyond the middle 

 descending towards the base, though this appearance may be deceptive. 

 In a much worn fragment of a large spine, which we have doubtfully 

 referred to this form (fig. 3, PI. XXII,) the lateral tubercles become 

 somewhat transverse in the lower anterior portion of the spine, recall- 

 ing the shape of the tubercles in the preceding form ; but the specimen 

 is generally so abraded, except along the anterior margin where the 

 strong denticles are still preserved, as to render the determination of 

 its identity with the present form unsatisfactory. 



The form here described strikingly resembles B. baculiformis, with 

 which it is associated, but may be recognized by its perhaps more taper- 

 ing and curved outline, less tumid apical extremity, more compressed 

 lateral surfaces, and the opposite direction of the oblique disposition of 

 the tubercles ; also the occurrence of strong tuberculose denticles along 

 the anterior border, as also in the upper portion of the posterior side, 

 offer marked contrast to the form cited above. 



Position and locality : Not common, in the fish-bed horizon of the 

 Upper Keokuk limestone, at Hamilton, and in the immediately superim- 

 posed geode bed at Warsaw, Illinois. 



Genus GAMPSACANTHUS, St. J. and W. 



Spines long, laterally compressed, tapering, curved backward (.?). 

 Lateral faces obscurely costate longitudiually, ornamented with more or 

 less regularly disposed, radiatingly sculptured or nearly smooth tuber- 

 cles, whose apices are directed upward, often interspersed with larger 

 tubercles of the same style, especially in the convex border ; posterior 

 or concave margin occupied by large, laterally compressed, subtrenchant 

 denticles, slightly curved downward, widely spaced, of nearly uniform 



