FOSSILS OF THE TOTSDAM GEOUP. 223 



rlor margin is elevated, forming a narrow, sliarply-rounded ridge, gradually 

 widening toward the outer end. The upper surface bears no markings 

 whatever, while the under side is marked by strong obliquely vertical striss. 



The pygidium associated with the other parts is transversely elliptical, 

 almost one-third wider than long; the anterior and posterior margins nearly 

 equally rounded, and the lateral extremities-, or sides, somewhat subangular 

 at the junction of the two lines; the posterior margin deeply and angularly 

 notched in the middle. Axial lobe small, about one-half as long as the shield, 

 elevated and subangular along the median line, and obscurely pointed behind; 

 marked by seven low, rounded rings, exclusive of the terminal ones. Lateral 

 lobes large, concavo-convex, rather rapidly declining on the inner half, and 

 slightly recurving to the margin ; marked by five broad, rather indistinct 

 ribs, exclusive of the anterior articulating projection, which become obsolete 

 near the middle of the lobe. 



The species is peculiar in its broad, expanded cephalic shield and rapidly- 

 tapering conical glabella; in these respects differing from all other forms 

 with which it is associated, as well as all of those from the Wisconsin beds 

 of this age, to so great an extent that there is not the least danger of mis- 

 taking it. In the broadly-expanded cheek and frontal limb, it resembles 

 some forms of Dikellocephahis, as well as in the form of the pygidium; but 

 the glabella is so very different that it at once distinguishes it, although so 

 closely allied to that genus in other respects. 



It closely resembles Fteroce^pJialus Sancti-sabm Roemer, from the same 

 position in Texas (see Kreid. von Texas, plate xi, fig. 1, p. 93), but differs 

 specifically in many and important features. 



Formation and locality. — In the lowest limestones seen on the west side 

 of Pogonip Mountain, White Pine, Nevada; age of the Potsdam group. Col- 

 lected by Arnold Hague, esq. 



Gyenus PTYCHASPIS Hall. 



FTYCnAEF fg PUSULOSA n. sp. ^ ^^ 



Plate II, fig. 27. 



Entire form unknown. Glabella and fixed cheeks united, irregularly 

 quadrangular in form, slightly angular in front, the margin declining on 



