224 THE CHICAGO ACADEMY OF SCIENCES. 



Dorsal furrows short, rather broad and shallow, starting from 

 pit-like depressions near the posterior margin of the cephalon, 

 converging anteriorly and terminating nearly opposite the 

 anterior extremities of the eyes but becoming less sharply de- 

 fined anteriorly. Eye-lobes sloping gently from the dorsal 

 furrows; eyes large, nearly horizontal in position, situated pos- 

 teriorly, describing nearly a semi-circle, from four to six times 

 as long as high; beneath the eyes the free cheeks are strongly 

 depressed, the surface extending horizontally for nearly one- 

 half their width, then curving abruptly downward and continu- 

 ing to the lateral margin in a gently convex but nearly vertical 

 curve. Posterior limb of the facial suture very short, anterior 

 limb describing a sigmoidal curve and intersecting the anterior 

 margin of the cephalon nearly in front of the eyes. 



The thorax and pygidium of this species have not been 

 recognized. 



The dimensions of a nearly perfect cephalon are: length 

 26.5 mm., width 47 mm., convexity 19.5 mm., width of cranidium 

 between the eyes 38 mm. 



Remarks. This species may always be distinguished from 

 its associates by the sub-elliptical outline of the cephalon, by 

 the posterior position of the eyes and by the conspicuous de- 

 pression of the inner portion of the free cheeks just below the 

 eyes. 



Localities. Near Lemont and at Joliet, Illinois. 

 Illaenus transversalis, n. sp., pi. xvi, figs. 7-9. 



Description^ Cephalon very short and broad, the convexity 

 nearly equal to the length, and the breadth more than twice 

 the length; anterior and lateral margins broadly rounded from 

 the dorsal to the ventral sides. Dorsal furrows moderately 

 impressed, short, scarcely reaching beyond the anterior margins 

 of the eyes. Eye-lobes sloping slightly from the dorsal furrows, 

 with a slightly convex surface. Eyes describing nearly a semi- 

 circle, nearly horizontal in position, their posterior extremities 

 approaching nearly to the posterior margin of the cephalon, 

 their surface convexly curved vertically and about five times 

 as long -as high. At the outer margins of the eyes, upon the 

 free cheeks, are slightly depressed, rounded furrows. Posterior 

 limb of the facial suture short, directed obliquely outward 

 toward the genal angles; anterior limb describing a slightly 

 sigrnoidal curve, cutting the anterior margin of the cephalon 

 almost directly in front of the eyes. Free cheeks broad, their dor- 



