176 SIXTEENTH REPORT OF THE CABINET OF NAT. HISTORY. 



This species is not uncommon in the friable greenish gray sandstone at 

 Trempaleau, and in some associated magnesian beds. It is readily distin- 

 guished from ail the other species by its narrow frontal 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. 



« 



GENUS ILL^NURUS ( n, g.). 



Body robust, broadly elliptical : head short, convex, semiellipti- 



cal, the front and sides regularly rounded and nearly straight 



behind. 

 Glabella subquadrate, convex, smooth, without distinct dorsal 



furrow : 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, narrow, subelliptical, 



convex in front and more curved behind. 



The similarity of this fossil to Ill^enus 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 differences, which, re- 

 garding its primordial associations, I have thought it worth while to con- 

 sider. The name -proposed sufficiently indicates its relations with Ill^nus. 



ILL^NURUS QUADRATUS (n.s.). 



plate VII. FIGS. 52-5?. 



Head broad, semielliptical. Glabella convex, a little longer than 

 wide, without visible occipital or dorsal furrow ; a scarcely 

 perceptible groove along the posterior margin ; the width a 

 little greater behind than before ; sides concave ; palpebral 

 lobes marginal, a little behind the middle of the glabella. 



Facial suture cutting the contour of the front a little within, or 

 nearly in, a vertical line drawn through the eye, and making a 

 very gentle concave curve to the outer margin of the palpebral 

 lobe. 



Movable cheeks scarcely longer than wide, with a deep sinus 

 below the middle, marking the form of the eye : posterior 

 extremities rounded, and straight upon the posterior margin. 



