THE NATURAL HISTORY SURVEY. 225 



sal surface nearly horizontally directed near the margin of the 

 eyes, then curving with a gentle convex curvature which be- 

 comes much more abrupt toward the anterior and posterior 

 margins. Genal angles obtusely rounded. 



Thorax and pygidium known not. 



The dimensions of the most complete cephalon observed 

 are: length 16 mm., width 40.5 mm., convexity 16 mm.,. width 

 of cranidium between the eyes 25 mm. 



Remarks. This species is easily distinguished from any of 

 its associates by the very broad and short form of the cephalon, 

 by its obtusely rounded genal angles, and by the broad rounding 

 of the anterior and lateral margins which extends even to the 

 posterior margin of the free cheeks. The species is not a com- 

 mon one, onjy a few incomplete heads having been observed. 



Locality. Bridgeport, Illinois. 



IHaenus imperator Hall, pi. xvi, figs. 13-16. 



Description. Cranidium strongly convex, wider than long, 

 sub-elliptical in outline. Glabella broad -and not denned in 

 front; dorsal furrows strong, straight, slightly c6nvergent an- 

 teriorly, less than one-half the length of the head. Cheeks con- 

 vex, their surface depressed below the glabella posteriorly and 

 becoming confluent with the general convex surface of the 

 head anteriorly. Eye-lobes small and situated close to the 

 posterior margin of the head, produced in a nearly horizontal 

 direction. Form of the free cheeks unknown. 



Pygidium sub-elliptical in outline, less convex than the 

 head, twice as wide as long, trilobate anteriorly; -the axial por- 

 tion occupying about one-third of the entire width along the 

 anterior margin, flattened on top, ill-defined posteriorly, the , 

 surface becoming confluent with the general convex surface of 

 the pygidium at about the middle of its length. 



Surface of both head and pygidium smooth. Thorax un- 

 known. 



The dimensions of a small cranidium are: length 20.5 mm.,, 

 width between the eyes 37 mm., convexity 20 mm. The 

 dimensions of a small but nearly perfect pygidium are: length 

 2 7. 5 'mm., width 55 mm., convexity 10.5 mm. 



Remarks. The dimensions given above are in both cases 

 those of small individuals. Larger specimens of heads attain- 

 ing a length of 80 mm. or more are not infrequently met with,, 

 as well as pygidia of similar proportions, but these larger speci- 

 mens are usually less well preserved. 



