226 THE CHICAGO ACADEMY OF SCIENCES. 



This species does not occur in the higher beds of the Nia- 

 garan limestone of the Chicago area, being most common in 

 certain beds at Joliet. It can be readily distinguished from all 

 other species of the genus in the Chicago area by reason of its 

 short and small eyes which are situated far back near the pos- 

 terior border, the dorsal furrows of the head are also stronger 

 than in most of the other species though in some they are much 

 longer, and the trilobation of the head along the posterior mar-: 

 gin is more strongly developed than in most of the species. The 

 pygidium in its broad and short form, with the anterior triloba- 

 tion, differs greatly from any of the other species. None of the 

 other Niagaran species from the Chicago area, when perfectly pre- 

 served, could possibly be mistaken for this one, but the heads 

 of /. ioxus, I. graftonensis with their short but strong dorsal 

 furrows, are often difficult to distinguish from /. imperator 

 when the free cheeks are removed and the eye-lobes injured. The 

 species much more closely resembles some of the Ordovician 

 members of the genus. It is really difficult to point out char- 

 acters whereby it can be readily distinguished from /. taurus 

 Hall of the Trenton limestone. The portions usually preserved 

 are also much like but very much larger than the same portions 

 of the little Thaleops ovatus of the Trenton limestone, but it 

 is not likely that the Niagaran species possessed the divergent 

 spine-like free cheeks of that species. 



Locality. Joliet, Illinois. 



Genus 3. ILLAENOIDES n. gen. 



Similar to Illaenus, but with the cephalon more strongly 

 trilobed, with longer and stronger dorsal furrows. The eyes 

 are small and situated much farther forward than in Illaenus. 

 Thorax and pygidium as in Illaenus. 



Illaenoides triloba, n. sp., pi. xvii, figs. 6-9; pi. xix, figs. 12-14. 



Description. Cephalon small or of medium size', semi- 

 elliptical in outline, rather strongly convex, anterior margin 

 sometimes slightly recurved but never extended into a pro- 

 jecting lip-like border. Dorsal furrows broad and strong, giving 

 to the head a conspicuous trilobate contour, extending nearly 

 to the anterior margin where they gradually disappear; glabella. 

 smooth, convex; fixed cheeks convex, pointed at their pbs- 

 tero-lateral extremities, the surface exclusive of the compressed 

 portion forming a pair of rather large, moderately convex, 

 sub-elliptical lobes; eye-lobes very small and short, scarcely 



