262 THE CHICAGO ACADEMY OF SCIENCES. 



laterally and posteriorly, are shallow, rounded, concave furrows. 

 Eyes small, situated opposite the second lateral glabellar lobes. 



Thorax consisting of thirteen segments, the length two to 

 two and one-half times that of the cephalon, strongly trilobed, 

 the axis about two-fifths the entire width, the pleural lobes 

 flattened on top for about one-third their width, then curving 

 abruptly to the lateral margins. The segments arched forward 

 on the axis, each one bearing a distinct, rounded node on either 

 side just within the dorsal furrows. The pleura of each segment 

 marked by a broad, deep, rounded furrow which originates at 

 the axial furrow and extends two-thirds or more of their length, 

 becoming shallower and' narrower dis tally and terminating on 

 the flattened distal portion of the segment as a slender groove 

 with a slight forward curvature. 



Pygidium irregularly sub-pentagonal in outline, strongly 

 trilobed. The axis tapering posteriorly with an obtusely rounded 

 extremity and terminating within the marginal border, marked 

 by six or seven annulations. Pleural lobes with a flattened 

 area in front, adjacent to the dorsal furrows, which tapers pos- 

 teriorly and becomes obsolete before reaching the end of the 

 axial lobe, outside this flattened area the surface slopes ab- 

 ruptly to the lateral margins ; marked by four or five segments 

 with no longitudinal depressions, which reach half way or less 

 than half way from the dorsal furrows to the margins; anterior 

 lateral angles abruptly flattened, lateral margins raised in a 

 rounded marginal border which becomes narrower posteriorly. 



The dimensions of a small, complete individual are: length 

 36 mm., greatest width across the posterior margin of the head 

 23 mm., length of head 10 mm., length of thorax 27 mm., 

 'length of pygidium 12.5 mm., width of pygidium 12.5 mm. 



Remarks. The discrepancy between the sums of the 

 lengths of the three regions of the specimen whose dimensions 

 are given above and the total length of the body, is due to the 

 fact that in the total dimension no account is taken of the cur- 

 vature of the dorsal surface of the body, while in taking the 

 three separate dimensions the measurements were made along 

 their median dorsal lines. 



This species is by far the commonest trilobite in the Niagaran 

 fauna of the Chicago area, and seems to be present in all horizons. 

 It is very commonly found with all portions of the body, head, 

 thorax and pygidium together, a condition of preservation which 

 is extremely rare in all other species of the fauna. 



