Contributions to Palceontology . 



167 



shallow groove, which leaves a thickened border of the 

 same width. 



This species is not uncommon in the friable greenish gray sand- 

 stone at Trempaleau, and in some associated magnesian beds. It is 

 readily distinguished from all the other species by its narrow/rontal 

 border, and the contraction just anterior to the eye. The movable 

 cheek differs from the others in the circular curving of its outer 

 limb. 



In its facial suture it approaches the Genus Arionellus, and 

 some separated thoracic segments in the same association are similar 

 to those of that genus. The character of the palpebral lobe, with 

 the large eye and form of cheek, are distinctive. 



GE1STJS ILL^EFUEUS ( n. g.). 



Body robust, broadly elliptical : head short, convex, 



semielliptical, the front and sides regularly rounded 



and nearly straight behind. 

 Glabella subquadrate, convex, smooth, without distinct 



dorsal farrow : palpebral lobe marginal. Cheeks wide. 

 Facial suture nearly vertical, slightly diverging anterior 



to the eye. 

 Movable cheeks wide and short. 



Thoracic articulations moderately convex, with a wide 

 central and narrow lateral lobe. Pygidium short, nar- 

 row, subelliptical, convex in front and more curved be- 

 hind. 



The similarity of this fossil to Illjentjs is manifest in all its 

 parts ; but the quadrate form and narrowness of the central part of 

 the head, and the great lateral extent of the cheeks, offer some dif- 

 ferences, which, regarding its primordial associations, I have 

 thought it worth while to consider. The name proposed sufficiently 

 indicates its relations with Ill^nus. 



