NO. 5 UPPER CAMBRIAN TRILOBITES RESSER 59 



bows outward slightly. Several pairs of recurved glabellar furrows 

 are visible. The occipital furrow, both on the glabella and fixigenes, 

 is deeply impressed. In cross section the cranidium is convex, reach- 

 ing the greatest curvature as the median line is approached. Longi- 

 tudinally the convexity is less, and the rate of curvature even through- 

 out. The brim width is a little more than one-fourth the glabellar 

 length, and is a simple convex band surr6unding the front of the 

 head. The associated pygidium is also convex, with a broad, faintly 

 outlined axis and fused pleural lobes. 



Davis formation; (loc. g2d) Federal Lead Mine No. 4, Flat River, 

 Missouri. 



Holotype. — U.S.N.M. No. 108740a; paratypes, Nos. io874ob-d. 



BYNUMINA MISSOURIENSIS, new species 

 Plate 10, Figures 23-26 



This is the more common Missouri species, and it differs from 

 B. caelata mainly in its narrower rim. 



B. missouriensis has a tapering glabella, but the curvature of the 

 lateral dorsal furrows is not even. It expands rapidly as far forward 

 as the first glabellar furrow, then suddenly begins to contract, main- 

 taining a nearly straight course until it approaches the anterior angles, 

 where the rate of taper becomes less again. Besides this peculiarity, 

 the species is characterized by a rim width of a little more than one- 

 fifth the glabellar length. 



Davis formation; (Iocs, iik, giy) Flat River, Missouri. 



Holotype. — U.S.N.M. No. 108741 ; paratypes, Nos. 108742a, b. 



STENOPILUS Raymond, 1924 

 STENOPILUS BACCA, new species 



Plate 10, Figures 8-13 



Such a featureless hemispherical trilobite as Stenopilus is difficult 

 to describe. Several species are here presented, chiefly to show the 

 differences between small and large examples, and the variation in 

 degree of sphericity. There seems to be little doubt but that these 

 trilobites developed from a Kingstouia ancestor. 



Large cranidia of S. hacca are nearly hemispherical in shape, but, 

 as may be seen in figures 10 and .11, small heads show a distinct 

 Kingstonia shape. Of course, the indentations of the dorsal furrow 

 in the rear remain, but unless the very convex specimen is specially 

 posed, they are obscured by the bulging cranidium. 



