222 THE CHICAGO ACADEMY OF SCIENCES. 



much less deeply impressed in their anterior extremities be- 

 yond the eyes, and it lacks the lip-like extension of the 'ante- 

 rior and lateral margins of that species. The cephalon differs 

 from /. grajtonensis in its greater convexity and in its more 

 nearly semi-circular outline. 



Locality. Bridgeport, Illinois. 

 Illaenus armatus Hall, pi. xviii, figs. 4-6. 



Description. Cephalon small and rather short, rather 

 strongly convex, semi-circular - in outline, the anterior and 

 lateral margins rounded and not produced into a marginal 

 lip-like extension. Dorsal furrows usually short and not 

 produced beyond the anterior extremities of the eyes, but in 

 some specimens they are continued anteriorly as faint depres- 

 sions and terminate in faintly denned pits. Eyes prominent, 

 their centers about the middle of the total height of the cepha- 

 lon, situated near the posterior margin. Eye-lobes convex, 

 extending in a nearly horizontal direction from the dorsal 

 furrows. Beneath the outer border of each eye, upon the free 

 cheeks, is a rather strong, rounded, depressed furrow. Free 

 cheeks convex, longer than wide, the genal angles produced 

 backward into rather short and blunt genal spines. Posterior 

 limb of the facial suture very short, anterior limb sloping ab- 

 ruptly to the antero-lateral margin. 



The dimensions" of a complete cephalon are: length 13 mm., 

 width 21 mm., convexity 9.5 mm., width of cranidium between 

 the eyes 17 mm. 



Remarks. This species is usually of smaller size than other 

 members of the genus in -this fauna, although specimens fre- 

 quently occur with greater dimensions than those of the speci- 

 men here illustrated. The species may always be recoginzed 

 by its short genal spines. 



Localities. Bridgeport and Lockport, Illinois. 



Illaenus ioxus Hall, pi. xviii, figs. 1-3. 



Description. Cephalon attaining a large size, sub-semi- 

 circular in outline, sub-truncate in front between the anterior 

 extremities of the facial sutures, moderately convex, the smaller 

 specimens more convex than the larger. Dorsal furrows rather 

 broad, but not deeply impressed, becoming obsolete beyond 

 the anterior extremities of the eyes. Eye-lobes depressed be- 

 low the median portion of the cephalon, sloping with a convex 

 curve from the dorsal furrows to the inner margins of the eyes. 



