244 Annals of the Carnegie Museum. 



at the posterior margin of the cephalon, but quickly become shallow 

 and fade out less than half way to the front. They converge slightly 

 for the greater part of their length, but at the anterior ends turn out- 

 ward a little. On the cast they are more strongly denned, wider, and 

 reach further forward and outward. Midway in their course they pass 

 through two lunate depressions, thus making a sigmoid curve, as 

 described by Billings in Illcenus americanus. The surface of the 

 cranidium is covered with punctae with the exception of the posterior 

 border. The punctae are especially strong on the glabella, but per- 

 fect specimens show four smooth, oval areas on either side of the 

 median line of this region. The arrangement of these smooth areas, 

 which is shown in Fig. 8, Plate LX., suggests the form of the gla- 

 bellar lobes of many trilobites. They probably indicate points of 

 attachment of muscles. 



The eyes are rather large, and are situated less than half their length 

 from the posterior margin of the cephalon. 



The thorax shows ten wide, flat segments. The axial lobe is rather 

 strongly arched, and over one third the total width of the thorax. 

 The pleura are flat for about one half their width, then abruptly 

 deflected. 



The pygidium is about as long as ihe thorax, somewhat rectangular 

 in outline, three-fifths as long as wide. Its most remarkable feature 

 is the abrupt truncation of the sides, which are at almost right angles 

 with the anterior margin. Axial lobe strongly convex on the anterior 

 margin, outlined by deep furrows at the sides, and by a shallow furrow 

 around the posterior end. The lobe is about half the length of the 

 pygidium. From the axial lobe radiate faint cracks, as in Illcenus 

 americanus. These cracks are formed by the confluence of the numer- 

 ous punctae. 



This species differs from Illainus americanus chiefly in the form of 

 the pygidium, which is much more strongly truncated at the sides, less 

 arcuate posteriorly, and has a more convex and prominent axial lobe. 

 The cephalon differs in that it is wider in proportion to the length, 

 and has shorter, straighter, and shallower dorsal furrows. 



Locality. — This species occurs in the Black River Limestone at 

 Tetreauville and Mechanicsville, near Ottawa, Canada, and also at 

 Pattersonville and Newport, New York. Probably many of the spec- 

 imens of Illaznus americanus reported from the Black River at various 

 localities really belong to this species. The cotypes are in the pri- 

 vate collection of Mr. Narraway. 



