46 PRIMORDIAL PERIOD. 



convexity of the head must have been considerable, and that the specimens 

 have been much flattened by pressiu-e. *■ When pei-fect specimens shall be 

 discovered, they may probably necessitate a modification of the foregoing- 

 description, but it is believed that such discovery will not necessitate material 

 change. The specimens of the collection are of various sizes, due to differ- 

 ence in age ; the transverse diameter of the head being from one and a half 

 to five and a half centimeters. 



This species is nearly allied to 0. Vennontana Hall, from strata in 

 Northern Vei-mont, generally refen-ed to the Potsdam epoch ; but it diff'ers 

 in many respects, the following being among the more important differences 

 that appear upon compaiing our specimens with the figure and original 

 description of that species: — 



The frontal lobe of the glabella of om* species does not reach the ante- 

 rior margin of the head by a space nearly equal to one-thu'd the length of 

 that lobe, instead of coming in contact with the frontal margin as in 0. Ver- 

 montana. The anterior ends of the eyes of our species reach forward nearly 

 into contact with the anterior lobe of the glabella, being considerably 

 farther forward than they are represented to reach in the figm-es of the 

 Vermont species. In our species, the raised marginal rim of the posterior 

 border of the head extends continuously across the occi]3ital lobe of the 

 glabella, but the figure and description of 0. Vennontana represent no such 

 raised rim. The posterior margin of the head of om- species is rounded 

 forward at the postero-lateral angles, while the figure of 0. Vermontana 

 represents the posterior border of the head as cm-ving backward, and forming 

 shai-p, spine-like angles with the lateral margins. The short cheek-spines 

 of that species are represented as widening at the base ; in ours, the cheek- 

 spines are longer, more slender, and do not thus widen. The long thu-d 

 plem-a of 0. Gilberti is not bent backward so abraptly as it is in 0. Ver- 

 montana, and the foimer species seems also to have reached a larger size 

 than the eastern one. 



Position and locality. — Strata of the Primordial period, probably of the 

 Potsdam epoch, Pioche, Nevada, and at Ophir City, Oquin-h range, Utah. 



