FOSSILS OF THE POTSDAM GEOUP. 231 



absence of all otber forms in the same block, has determined us in so doing. 

 The axial lobe forms more than one-third the width of the shield, is short 

 and rounded obconical; marked by a node at its upper end, and divided 

 across by a doubly-curved transverse furrow near the lower end. Dorsal 

 furrows distinct, and continued as a median furrow from the end of the axial 

 lobe to the posterior margin. Posterior border emarginate and surrounded 

 by a flattened rim 



Formation and locality. — In limestones of the Potsdam group, at Eureka, 

 Nevada. Collected by Arnold Hague, esq. 



Agnostus tumidosus n. sp. y 



Plate I, fig. 32. 



Cephalic shield highly dome-shaped in outline, very slightly contracted 

 near the occipital border, very convex, and margined by a narrow, flattened 

 border. Dorsal furrows very distinct, not continued in front beyond the 

 glabella. Glabella proportionally small, less than two-thirds as long as the 

 head, distinctly conical in form and very highly convex, especially tumid 

 in the lower part; the central tubercle marked near its edge by a very slight, 

 depressed line, which presents the appearance of a border surrounding it. 

 Anterior lobe forming a little more than one-fourth of the length of the 

 glabella; the furrow separating it from the principal lobe very distinct. 

 Occipital nodes very small, triangular, and distinctly modifying the base of 

 the glabella on each side. Thoracic segments and caudal shield unknown. 



The specimen above described is so entirely distinct and well marked 

 that we have no hesitation in giving it as a species, although it is as yet 

 the only individual specimen noticed. The highly dome-shaped outline, 

 literally horseshoe-shaped, is peculiar, which, together with the form and 

 markings of the glabella, cannot fail to distinguish it from all others. 



Formation and locality. — In limestones of the Potsdam group, at Eureka, 

 Nevada. Collected by Arnold Hague, esq. 



