GENUS ILLiENUS. 



331 



Compared with Illcenus {Bumastus) barriensis, the head is more promi- 

 nent in the middle, and more produced in front ; while the eyes are more 

 prominent. The more conspicuous difference, however, is the prolongation 

 of the posterior angle of the cheek into a short strong spine, a feature 

 which I have not observed in I. barriensis. The pygidium is also more 

 nearly semicircular, being broader in proportion to its length. 



Formation and locality. In limestone of the Niagara group at Bridge- 

 port, Illinois, and at Grafton and Racine, Wisconsin. 



ILLiENUS INSIGNIS (n. s.). 



Head large ; glabella prominent and somewhat regularly arcuate from 

 front to base ; anterior border with the margin a little recurved. Dor- 

 sal furrows distinctly marked from the base of the head for three- 

 fourths the distance to the anterior margin, where they terminate in a 

 distinct rounded pit ; palpebral lobe large, elongate, the eye being situ- 

 ated at some little distance from the posterior margin of the head. Fa- 

 cial suture running out on the anterior border within the line of the eye. 

 The full extent of the cheek is not known. 



The form of the glabella and the convexity of a single articulation of the 

 thorax indicate the general form to have been very convex. 

 The pygidium is parabolic, very convex ; about as long or a little longer 

 than wide. Anterior margin nearly straight along the middle for about 

 half the width, for the attachment of the axis of the thorax, and abruptly 

 receding towards the sides. 



The specimens are mostly casts of the interior, but the species is readily 

 recognized by the elongate and regularly arcuate form of the glabella, and 

 the strongly marked dorsal furrows, and in these respects it differs conspicu- 

 ously from any other species in the formation. A single specimen partially 

 preserving the crust does not show the glabellar furrows so distinctly as 

 the casts. 



5 6 



-'./ 



Fig. 5. I. insignis. The liead, showing the dorsal furrow and direction of the facial 

 suture. 



Fig. 6. A profile view of the head. <,- 



