PALEONTOLOGY OF ILLINOIS. 



marginal furrow, which becomes suddenly obsolete on 

 reaching the anterior lateral margins of the glabella, and 

 extends backward to or even a little upon the posterior lat- 

 eral, sub-spiniform appendages; posterior margins with an 

 elevated rim, strongly defined by the deep continuation of 

 the neck furrow; lateral margins showing, as seen from 

 above, a narrow rim, which, in a side view, is seen to be 

 deep, vertically flattened, and marked by fine, parallel, 

 longitudinal stria?; anteriorly it continues around the front 

 of the glabella, but does not project so as to be visible from 

 above, while its upper margin is continued in the form of 

 a carina along the middle of the posterior lateral spines to 

 their points. Facial sutures cutting the anterior border 

 in front of the eyes, and the posterior margins of the 

 cheeks behind the outer margins of the eyes. Thorax 

 nearly as long as the head, but somewhat narrower, very 

 distinctly trilobate; mesial lobe prominent, rounded, and 

 a little wider than the lateral lobes; its nine segments 

 narrow and sub-angular. Lateral lobes depressed and flat- 

 tened near the mesial lobe, and so abruptly sloping from 

 the outer side of this flattened space, as to impart a slight 

 angularity along that region ; segments corresponding 

 in size with the segments of the mesial lobe, and dis- 

 tinctly kneed near the middle, outside of which they are 

 bent down and obliquely flattened for folding together, and 

 rounded at the extremities. Pygidium very convex, smaller 

 than the cephalic shield, forming more than a semicircle, 

 with anterior lateral angles obliquely truncated; posterior 

 outline regularly rounded, with a moderately wide, smooth, 

 depressed, nearly flat or sloping marginal zone ; trilobatioii 

 as in the thorax, strongly denned; mesial lobe prominent, 

 as wide anteriorly as one of the lateral lobes, including its 

 border, distinctly flattened on each side, slightly tapering 

 to an obtuse termination, less than half its own greatest 

 anterior breadth from the posterior edge, segments eleven 

 or twelve, well denned above, but nearly obsolete on the 



