THE NATURAL HISTORY SURVEY. 279 



rhomboidal or sub-triangular in outline; first and second lateral 

 lobes confluent at their outer margins, the first lobes sub- 

 triangular in outline, the second sub-circular or sub-elliptical, 

 but little more than half the size of the first ; third lateral lobes 

 much smaller than the second, wider at their outer extremities 

 than within; first lateral furrows the longest, broad and of 

 moderate depth, straight, extending obliquely inwards and 

 backwards from just in front of the anterior margins of the 

 eyes, not joined across the median portion of the glabella, the 

 width of the unfurrowed median region being one-half the width 

 of the glabella along its posterior margin ; second lateral furrows 

 nearly transverse in direction, situated opposite the posterior 

 third of the eyes, not joined across the glabella, their inner 

 extremities lying directly behind the inner extremities of the 

 anterior furrows, they are deeply impressed within and, in the 

 casts, are joined with the dorsal furrows externally by very 

 slight depressions; third lateral furrows deeply impressed, their 

 inner extremities curving slightly forward and terminating 

 directly behind the inner extremities of the two anterior pairs 

 of furrows. Occipital furrow narrow, most deeply impressed 

 laterally, arched forward in its median portion. Occipital 

 segment broadest in the middle. Cheeks rather large, sub- 

 triangular in outline, genal angles produced into short spines, 

 the lateral and posterior margins with a raised marginal border 

 which is rounded laterally but becomes sharply elevated poster- 

 iorly as it approaches the axial portion of the cephalon; the 

 marginal furrows within the raised marginal borders are deepest 

 and most sharply defined posteriorly behind the eyes, they are 

 shallower and broader as they round the genal angles and then 

 become a little deeper in the middle portion of the lateral mar- 

 gins; within the marginal furrows the surface of the cheeks is 

 moderately convex. The palpebral lobes elevated at their 

 outer margins to the height of the surface of the glabella; the 

 eyes are large and prominent, their length being about five- 

 twelfths the length of the cephalon along its median line, and 

 the height of the faceted portion three-fifths their length. 

 Thorax and pygidium not recognized. 



The dimensions of a nearly complete head are : length along 

 median line 24 mm., width at extremities of genal spines 31.5 

 mm., maximum width of glabella 16 mm., width of glabella 

 posteriorly 9.5 mm. 



Remarks. This species is readily distinguished from any 

 other members of the genus in the Chicago fauna by reason of 



