FOSSILS OF THE LOWER SILURIAN. 



97 



There are but slight traces of the fixed cheeks attached to the glabella. 

 The species is characterized by the peculiarly shaped glabella. 



Formation and locality. Pogonip Group, on the west slope of McCoy's 

 Ridge, Eureka District, Nevada. 



Genus ILL^NURUS Hall. 



Illaenurus Eurekensis, n. sp. 

 Plate xii, fig8. 4, 4 a. 



Head of medium width, semi-elliptical; glabella subquadrate, convex, 

 a little longer than wide, as 5 to 4, with scarcely perceptible occipital or 

 dorsal furrows; palpebral lobes nearly central on the margins; frontal mar- 

 gin nearly straight; free cheeks a little longer than wide, with the base of 

 the eyes cutting into them about the middle of the inner margin; genal 

 angles broadly rounded. 



The facial suture, cutting the frontal rim on a line with the outer edge 

 of the eyelobe, passes directly to the anterior margin of the eyelobe; it 

 encircles that and then curves gently outward to the posterior margin, form- 

 ing a short, triangular-shaped lateral limb. 



Surface minutely punctate. 



The pygidiae associated with the head parts vary very much. Two 

 are illustrated on plate xii, figs. 1 2 and 1 9, neither of which has any re- 

 semblance to the pygidium of Illsenurus. Another, discovered since the 

 plate was prepared, shows a short, smooth axial lobe, slight dorsal furrows, 

 and gently convex, broad lateral lobes; this is probably the pygidium of 

 this species, as numerous examples of the glabella occur on the same frag- 

 ment of rock. 



In the fourth volume of the Geology of Wisconsin a species of Illae- 

 nurus, I. convexus, is given as occurring in the Lower Magnesian limestone 

 of that State, and the genus is also represented in the Potsdam sandstone, 

 Illoenurus quadratus Hall. In Nevada as yet it is only known from the 

 horizon above that representing the Potsdam, or the correspondent of the 

 Calciferous formation of the New York geologists. 



Formation and localities. Lower portion of the Pogonip Group, on the 

 east slope of the ridge east of the Hamburg Ridge; on second ridge south- 

 7 o D w 



