GENEEAL 0B8EEVAT10NS. 7 



LOWER SILURIAN. 



The Lower Silurian age is represented in these collections by fossils 

 referable to all tlu'ee of its periods, namely, the Primordial, Canadian, and 

 Trenton. 



PUIMOI.'DIAL PERIOD. 



The collections contain fossils of the Primordial period that were 

 obtained in Western Utah, Southeastern Nevada, and Western Ari::ona, all 

 of which are probably referable to the Potsdam epoch of that period. The 

 most conspicuous feature of the fauna of the period, as represented by these 

 collections, consists in the preponderance of the Articulate over all other 

 forms of life, and which is represented by Trilobites alone, with the probable 

 exception of the tracks described on a subsequent page. This apparent 

 preponderance, however, is likely to be greatly modified by future discov- 

 eries. 



The genera of Trilobites which these localities have afforded are 

 Olenellus, ConocorypJie, Agnostus, and Asapliiscus ; the latter being a new 

 genus, recently proposed by Mr. F. B. Meek, probably allied with the 

 Asaphidce, while the others, as well as the geological horizon they are 

 understood to characterize, are well known. 



The discovery of two species of Olenellus at Pioche, Nev., is especially 

 important and interesting, since that genus has hitherto been regarded as 

 distinctively characteristic of a well-defined Primordial horizon in America. 

 The interesting nature of the discovery is increased by the fact that, in their 

 general characteristics, these two Nevada species of Olenellus respectively 

 represent two other well-known species of that genus similarly associated 

 in strata of the same period in Vermont and Canada. While all the other 

 strata herein referred to the Primordial period have been so refen-ed almost 

 wholly upon paleontological evidence, the Tonto shale of the Grrand Canon 

 of the Colorado River has been referred to that period by the geologists of 

 the expedition largely upon stratigraphical evidence. The only indications 

 of life that these last-named strata have afforded to the collections consist 

 of two species of Oniziana, besides the tracks that have just been referred to 

 It is true that the presence of Cruziana in those strata does not prove their 

 Primordial age ; but, as such fomis are rarely found in strata of other periods. 



