SILURIAN SPEOIES. 23 



is the greater length of the posterior kiteral spines of the cephahc, shield in 

 Dr. White's specimens, which ditiereiice is prol)ably sexual; Avhile the others 

 seem to be mainly due to the accidental flattening of our specimen. 



LocaUtij and position. — Antelope Springs, House Range, Utah ; LoAver 

 Silurian, and probably, judging from the known position of the genus Cono- 

 cephalites in the rocks of this country and Europe, from the Primordial 

 Zone. 



Genus PARADOXIDES, Brongniart. 



Paradoxides? Nevadensis, Meek. 



Plate 1, fig. .'). 



Para^oxides ? Nevadensis, Meek (1870), Proceed. Acad. Nat. Sci. Pbilad., 62. 



The only speciiiien of this Trilobite obtained consists of a natural cast, 

 formed by a moderately thick crust of an-agonite, deposited in a natural 

 mould, or impression, of a part of the thorax and the pygidium, with the free 

 borders of the latter broken away. Its rather large size, much depressed 

 form, spiniferous pleuni?, and general physiognomy, as far as seen, at once 

 recall to the mind the well-known genu.s Paradoxides. A closer inspection, 

 however, shows its pygidium to be proportionally larger than we see in the 

 known species of that genus, with possibly the exception of P. Forschham- 

 meri of Angelin. 



Of the thorax, eight of the posterior segments are preserved. These show 

 the axial lobe to be much depressed, and about as wide as the lateral ones, 

 exclusive of the free recurved points of the pleura?. The segments of the 

 axial lobe are defined by a broml, rounded furrow, or depression, across the 

 anterior side of each, and have nuich the general aj^pearance of those of 

 some species of Paradoxides, being a little thickened, squarely truncated, 

 and shghtly curved forward at the ends. But they differ in showing dis- 

 tinct remains of a mesial spine, or tubercle, on each, and in having an obscure, 

 oblique furrow, or depression, on each side, passing outward and backward 

 from the Inroad, anterior, transverse furrow to the posterior lateral angles, so 

 as partly to isolate the slightly-thickened and truncated extremities of each. 

 The lateral lobes are nearly flat, and composed of pleurte that extend straight 

 outward at right angles to the axis, to their free extremities, which are 



