22O THE CHICAGO ACADEMY OF SCIENCES. 



posteriorly, becoming stronger towards the front, they originate 

 at the posterior margin back of the eyes and describe a curve 

 above the eyes terminating in rounded pits about half way 

 between the eyes and the anterior margin of the cephalon; 

 below the eyes are other curved furrows originating near the 

 base of the dorsal furrows with their anterior extremities nearly 

 meeting those of the dorsal furrows, so that each eye is nearly 

 surrounded by a sub-elliptical depression. The eyes are of 

 moderate size and are situated back of the middle line of the 

 head. 



Thorax consisting of ten short 'segments with no indication 

 of dorsal furrows. 



Pygidium semi-elliptical in outline, not trilobate, strongly 

 convex, a little flattened on top, becoming concave near the 

 margin posteriorly so as to form an ill-defined concave border, 

 which extends to the middle of the lateral margins and then 

 disappears. 



The dimensions of an imperfect specimen are: length 56 

 mm., width 32 mm., length of cephalon 20 mm., convexity of 

 cephalon 13 onm., length of pygidium 21.5 mm. 



Remarks. In the original description of this species by 

 Whitfield, the specific name niagarensis was used, the reason 

 for changing it to madisonanus in a later publication by the 

 same author not being apparent, since the first name used had 

 not been preoccupied. The entire description originally pub- 

 lished for /. niagarensis was copied without change as the de- 

 scription of /. madisonanus. If the law of priority be followed 

 it will be necessary to go back to the original specific designa- 

 tion of the species, /. niagarensis, as is done in the present Bul- 

 letin. 



The head of this species with its elongate dorsal furrows 

 terminating in rounded pits, somewhat resembles /. insignis, but 

 it may always be distinguished from that species by the ab- 

 sence of the projecting lip-like marginal border in front and by 

 its less convexity; the outline of the head is also different from 

 that species, it being more nearly sub-elliptical. The pygidia 

 of the two species are very different, that of /. niagarensis may 

 be recognized by the absence of the slight median ridge and 

 by the concave border. 



Localities. Joliet and Lockport, Illinois. 

 Illaenus chicagoensis, n. sp., pi. xvi, figs. 10-12. 



Description. Cranidium of medium size, semi-circular in 

 outline, strongly convex, the convexity usually a little greater 



