THE NATURAL HISTORY SURVEY. 227 



separated from the general surface of the fixed cheeks, situated 

 near the middle of the total length of the head; posterior limb 

 of the facial suture nearly straight, intersecting the posterior 

 margin near the rounded genal angle; anterior limb shorter 

 than the posterior, intersecting the anterior margin nearly in 

 front of the eyes. Free cheeks longer than wide, sub-triangular 

 in outline, the surface convex except beneath the eyes where 

 there is a rounded furrow nearly horizontal in direction. 



Pygidium sub-elliptical in outline, moderately convex, 

 with the greatest convexity near the center, axial furrows want- 

 ing, thedateral borders somewhat depressed. 



The dimensions of a rather small but nearly perfect cepha- 

 lon are: length 18 mm., width 26 mm., convexity II mm., 

 width of cranidium between the eyes 18.5 mm. The dimen- 

 sions of a large pygidium are: length 29 mm., width 36 mm., 

 greatest convexity 11 mm. 



Remarks. This species is one of the rarer members of the 

 fauna of the Chicago area, and can always be distinguished from 

 members of the allied genus Illaenus by its more strongly trilobed 

 head, and its small eyes which are situated much farther for- 

 ward than in any of the associated species of that genus. 



Localities. Bridgeport, Joliet and Chicago Drainage Canal, 



near Lemont. 



Family 3. PROETIDAE Barrande. 



"Cephalon about one-third of the whole animal; genal 

 angles generally produced into spines; glabella tumid, with two 

 lateral basal lobes defined by oblique furrows in front of the 

 neck segment. Free cheeks large, separate. Sutures extending 

 from the posterior margin inward to the eyes, and then forward, 

 cutting the anterior margin separately. Eyes usually prominent, 

 often large. Thorax of from eight to twenty-two free segments, 

 with grooved pleura. Pygidium usually of many -segments ; pleural 

 and axial portions strongly grooved; margin entire or dentate." 

 Beecher, Zittel-Eastman Text Book of Paleontology, p. 631. 



Genus 4. PROETUS Stein., 1831. 



Complete body sub-elliptical irr outline. Cephalon semi- 

 circular, with a thickened marginal border; glabella well de- 

 fined by the axial furrows, more or less strongly convex, narrower 

 and rounded in front and usually extending to the inner side 

 of the marginal border, lateral furrows obsolescent, basal lobes 

 often present, occipital furrow well marked; eyes large and promi- 

 nent, placed near the glabella. Thorax with well defined axis, 



