FOSSILS OF THE NIAGARA GROUP. 421 



and 4. None of these differences, however, appear to be very important 

 or strongly marked. On the head the eye is placed a little more 

 obliquely ; the movable cheek is comparatively longer from the posterior 

 angle to its anterior margin ; the facial suture lines unite with the rostral 

 suture, forming an abrupt angle instead of a rounded one ; the rostral 

 shield is proportionally narrower from side to side, especially on the 

 inner margin, where it unites with the hypostoma. 



The differences in the pygidium are not readily observed. -, We have 

 no means of comparing the articulations of the thorax in the Wisconsin 

 specimens. 



The New York specimens usually referred to /. harriensis have the 

 same form of head and pygidium as those of Wisconsin, and differ from 

 j,jg j2 the figures of the British species in the 



direction of the terminations of the pleura. 

 Notwithstanding the differences are not con- 

 spicuous, I am inclined to regard them as 

 of the same importance as those which dis- 

 tinguish closely allied species. 



The form of the rostral shield and the 

 direction of the suture lines are shown in the accompanying fig. 12. 



This species is of common occurrence in the Niagara group of Wiscon- 

 sin, at Racine, Waukesha, Wauwatosa, and other places. 



It usually occurs as separated heads and pygidia, with detached por- 

 tions of the thorax. It has sometimes attained a very large size, the 

 head being two and a half inches in length ; equalling in size the largest 

 head figured from the Niagara shale of New York. 



iLLiENUS CUNICULUS, N. S. 

 PLATE XXII, FIG. 12. 



Glabella subquadrangular in outline, broadly rounded on the anterior 

 margin, with the edge sharply recurved ; general surface regularly 

 convex, a little more arcuate transversely than in a longitudinal 

 direction ; length and breadth very nearly equal. Palpebral lobes 

 moderately elevated, somewhat triangular in form, and laterally 

 produced ; situated very near the occipital border. Suture line 

 reaching the posterior margin of the head, a little within the outer 

 angle of the eye lobe, and slightly indented in front of the eye ; 

 thence directed toward the anterior margin with a slightly sigmoid 



