THE NATURAL HISTORY SURVEY. 22Q 



V 



furrows and then Becoming moderately convex to the lateral 

 margins, the thickened marginal border narrow and scarcely 

 differentiated from the general surface, becoming a little wider 

 posteriorly; each slope is divided into nine segments which be- 

 come faint posteriorly, the last two being scarcely recognizable, 

 the anterior-most segments extend entirely 'to the lateral margin, 

 but the posterior ones terminate within the margin, in some 

 specimens the broader segments are faintly grooved but in others 

 no grooves can be detected. 



The surface of both head and pygidium are smooth. 



The dimensions of a nearly complete head with the excep- 

 tion of the genal spines and occipital segment are: width 15.5 

 mm., length along median line 9 mm., width of glabella at 

 occipital furrow 6.5 mm. The dimensions of a very perfect 

 pygidium are: width 10 mm., length 6.5 mm., width of axis at 

 anterior margin 3 mm., length of axis 5.5 mm. 



Remarks. This little trilobite has been observed only in a 

 brown limestone near Channahon, Will County, Illinois, on the 

 Des Plaines river, which is believed to be the lowest Niagaran 

 horizon in the Chicago area. This horizon may be the same 

 as the so-called Clin,ton of Ohio and Indiana. The species 

 occurs only as detached, more or less fragmentary, heads and 

 pygidia. This species most closely resembles P . determinatus 

 Foerste, from the Ohio Clinton formation but it may be distin- 

 guished from that one by its more prominent glabella, especially 

 in front, and by the more strongly impressed dorsal furrows in 

 front of the glabella, and by the more strongly impressed lat- 

 eral glabellar furrows. 



Locality. Channahon, Will County,. Illinois. 



Proetus ? handwerki, n. sp., pi. xx, figs. 8-9. 



Description. Head crescentiform in outline, its. width 

 between the extremities of the genal spines nearly three times 

 its length along the" median line. Glabella longitudinally ( semi- 

 elliptical in outline, bounded by well-defined dorsal furrows* 

 truncated posteriorly, broadly rounded, strongly convex and 

 protuberant in front; frontal lobe large and almost hemispher- 

 ical; two pairs of lateral lobes may be detected but they are 

 very small ; of the two pairs of lateral furrows the first are very 

 faint and short, being little more than slight indentations upon 

 the lateral surfaces of the glabella at about the middle *of its 

 length, the second or posterior pair are a little more deeply im- 

 pressed and extend from the dorsal furrow inward and then 



