SILUKIAN SPECIES. 23 



is the greater length of the posterior Literal spines of the cephalic shield in 

 Dr. White's specimens, which difference is probably sexual; while the others 

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



Locality and position. — Antelope Springs, House Range, Utah ; Lower 

 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. 



Paradgxides "^ Nevabensis, Meek. au^ C~ 



Plate 1, fig. .5. 



Faradoxides? Nevadensis, Meek (1870), Proceed. Acad. Nat. Sci. Philad., 62. /o ^ ^ [j^ ^'^^ ^*'^^ 



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

 formed by a moderately thick crust of arragonite, 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 pleurae, and general physiognomy, as far as seen, at once 

 recall to the mind the well-known genus 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. ForscJiJiam- 

 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 pleurte. The segments of the 

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

 anterior side of each, and have much the general appearance of those of 

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

 and slightly 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 broad, 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 pleurse that extend straight 

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



