260 PALEONTOLOGY OF ILLINOIS. 



the Keokuk form, Pnigeacanthus pnigeus l (N. and W., sp.), from 

 which it is distinguished by its somewhat stouter proportions, less 

 deeply concave posterior border, and the peculiar tuberculation of 

 the borders. The general character of the tuberculation of the 

 lateral surfaces is much the same in both forms, but in the present 

 one the tubercles are relatively smaller and less crowded. 



Geological position and locality: St. Louis limestone ; Alton, Illinois. 



i. We are under obligations to S. A. Miller, Esa-, for reminding us of our remissness 

 in needlessly adding to the already burdensome complications of nomenclature, which 

 we perpetrated in abandoning the original specific designation applied to this fossil when 

 we transferred it to a distinct genus, of which it is the type. 



