NO. 5 UPPER CAMBRIAN TRILOBITES RESSER 39 



from the Upper Mississippi Valley. On exfoliated cranidia the dorsal 

 furrows show faintly for a short distance anterior to the eye, touch- 

 ing both ends of the eye lobe. Exfoliated specimens have a median 

 eye situated about one-third the distance forward from the rear 

 margin. In front of the eyes, which are situated behind the middle 

 of the cranidium, the facial suture diverges considerably to form wide 

 anterior angles. The thickened, heavily striated rim is almost vertical 

 to the horizontal plane of the cranidium. In cross section the cranidium 

 has a slight even curvature, but longitudinally it has considerably more 

 relief, also attained by even curvature. 



The libragene forms nearly a quarter circle, with its convexity 

 similar to that of the cranidium. The heavily striated border con- 

 tinues around the margin, decreasing toward the genal angle, which 

 evidently bore no spine. 



The pygidium is short, very wide, and lacks differentiation of axis 

 or pleura. At the anterior margin two slight depressions indicate the 

 presence of a very wide axis. The pygidium evidently came to rather 

 sharp points at its lateral angles. 



Portions of six simple thoracic segments remain articulated on one 

 rock fragment. 



Lyell formation ; (loc. 2od) Tilted Mountain Brook, 9 miles east 

 of Lake Louise, Alberta. 



Holotype. — U.S.N.M. No. 108690a; paratype, No. 108690b. 



ILLAENURUS (?) SINCLAIRENSIS, new species 

 Plate 6, Figures 13-15 



This species, doubtfully referred to lUacnurus, comes from some- 

 what older beds than those that contain the more characteristic species. 

 The cranidium is wider than long and exclusive of the posterolateral 

 limbs is rectangular in outline. No trace of dorsal furrows has been 

 observed. In cross section this species is rather flat, but longitudinally 

 it is considerably convex, with the anterior margin of the cranidium 

 turned under. A peculiar feature is the development of a rim like 

 that of Platycolpus. A small occipital tubercle suggests the presence 

 of a medial eye, and short shallow multiple furrows near it evidently 

 represent the occipital furrow. The eyes are of moderate size, situ- 

 ated slightly forward of the median point of the cranidium. 



Sabine formation ; (loc. i6t) Sinclair Canyon, Brisco Range, British 

 Columbia. 



Holotype. — U.S.N.M. No. io869ra; paratypes, Nos. 108691b, c. 



