NO. 5 UPPER CAMBRIAN TRILOBITES RESSER 87 



cranidium. A peculiar feature of the libragene is the convexity that 

 begins about halfway back in the rim and extends into the round, 

 elevated genal spine. 



Davis formation; (loc. iik) Federal Lead Mine No. 4, Flat River, 

 Missouri. 



Holotype.—U. S.'N.M. No. 108798a; paratypes, Nos. io8798b-d. 



IDDINGSIA BICINCTA, new species 



Plate 16, Figures 27-29 



Several plump cranidia from Blue Creek Canyon constitute another 

 species. Compared with others in the region the distinctive feature 

 seems to be the subdivision of the brim into two approximately equal 

 portions. The rather short glabella is rounded in front and rises above 

 the dorsal furrow with an even curvature. Furrows are so faint that 

 they are scarcely traceable. The occipital furrow on the other hand is 

 wide and deep, and the ring carries a long slender spine. The brim 

 width nearly equals the length of the glabella exclusive of the neck 

 ring. At the anterior end of the eye, the fixigenes are only one-third 

 the glabellar width. Divergence of the facial suture forms rather 

 large anterior angles. Longitudinally the cranidium is convex, the 

 rear half of the glabella being nearly level and the anterior third 

 sharply curved, and the slightly convex preglabellar area continues 

 its downward trend, while the rim turns to a nearly horizontal position. 

 In cross section the glabella appears elevated with a nearly circular 

 curvature. In like manner the fixigenes near the eyes are also convex. 

 There is but a slight depression of the anterior angles so that the 

 slope of the preglabellar area is not much more at the sides than in 

 the middle. Surface of test nearly smooth. However, the exfoliated 

 preglabellar area is marked by rather strong vertical anastomosing 

 lines. 



Honey Creek limestone ; (loc. gq) Blue Creek Canyon, Wichita 

 Mountains, Oklahoma. 



Holofypc.—U.S.N.M. No. 108799. 



IDDINGSIA CRASSIMARGINATA, new species 

 Plate 16, Figures 30-32 



This cranidium, associated with /. missouricnsis, is very distinct, 

 owing to its wide, flaring brim and relatively small glabella. The 

 glabella tapers to a slightly rounded front and has two pairs of 

 furrows. The brim is nearly as wide as the length of the glabella 

 exclusive of the occipital ring. It has a wide, slightly concave rim 



