.-^r 



228 PALEONTOLOGY, 



along the median line, reaching two-thirds of the entire length of the shield; 

 obtusely pointed at the extremity, and marked by five transverse rings, 

 exclusive of the terminal ones, which gradually decrease in size poste- 

 riorly. Lateral lobes moderately elevated and convex on the inner half, 

 becoming concave or flattened toward the edge ; marked by four elevated, 

 angular ribs on each side, including the one submarginal to the anterior 

 border, with broad, shallow, depressed areas between them. The ribs rise 

 from the margin of the axial lobe, and reach the border of the shield at the 

 angles of the marginal sinuosities, most strongly marked near the axis, and 

 becoming nearly obsolete toward the outer border. Surface of the crust 

 apparently smooth. 



The specimen is remarkable for the simple ribs of the lateral lobes, 

 and for the digitation of the margin. In these respects, it differs very 

 materially from any Trilobite hitherto described from the Potsdam group; 

 and it is possible that it may have come from a somewhat higher position 

 at the same locality than the other specimens with which it is placed. 



Formation and locality. — In dark crystalline limestone of the Potsdam 

 group, on the west side of Pogonip Mountain, White Pine District, and in 

 the Eureka District, Nevada. Collected by Arnold Hague, esq. 



Genus AONOSTUS Brongniart. 

 Agnostus communis n. sp. 



Plate I, figs. 28-29. 



Cephalic shield subparaboloid, wider than long, the respective diameters 

 being as six and seven. Surface strongly convex and distinctly trilobed. 

 Glabella nearly equaling one-third of the width of the shield, more promi- 

 nent than the lateral lobes, and separated from them by distinct dorsal fur- 

 rows, three-fourths as long as the entire length of the head, broadest at base 

 and gradually narrowing for two-thirds of the length, beyond which point 

 it is more abruptly narrowed or obtusely pointed; anterior third separated 

 from the part behind by a faint, scarcely perceptibly impressed, transverse 

 line. Central portion of the glabella marked by a distinct elongated and 

 angular tubercle. Right and left lateral lobes, or sides of the head, sepa- 

 rated in front of the glabella by a narrow furrow, which is a continuation 



