﻿C. D. Walcott — Cambrian System of North America. 151 



Upper Cambrian horizon, leaving but four genera that are 

 alone common to the Middle and Lower horizons. One genus, 

 Dendrograptus, is doubtfully identified in the Paradoxides 

 horizon of New Brunswick that occurs in the Upper Cam- 

 brian, and is, as yet, unknown in the Middle Cambrian. The 

 genus Agraulos is also found in the Lower and Upper, but 

 not in the Middle Cambrian. Of species, not one of the 76 

 of the American Lower Cambrian fauna are known to occur 

 in the Middle Cambrian fauna, which, with its 107 species, 

 stands out clearly from the older fauna and also from the more 

 recent Potsdam fauna, as but three of its species, Prolospongia 

 fenestrate Stenoiheca elongata and Acrotreta gemma, are known 

 to occur in the Upper Cambrian, and 16 of the genera in the 

 Middle Cambrian are not known to pass up into the Upper 

 Cambrian or into the Lower Silurian (Ordovician) faunas. Not 

 one species is known to be common to the Lower and Upper 

 Cambrian horizons. 



Having studied the Middle Cambrian fauna more thoroughly 

 than that of the lower and upper horizons, I will speak of it 

 on that account and, also, from the fact that its character and 

 geographic distribution is not as well known as the other two. 



As a whole, we notice that it combines the characters of the 

 Lower Cambrian and Upper Cambrian faunas and yet is dis- 

 tinct from each of them. There does not appear to be an 

 equivalent fauna in the Cambrian system of Europe either in 

 Bohemia, the Scandinavian area, or in Wales; but from the 

 Island of Sardinia, Dr. Bornemann has described a group of 

 sponge-like bodies closely related, if not identical with Ethmo- 

 phyllum and Archceocyalhus of the American Middle Cambrian 

 fauna; he also names Kuiorgina cingulata which is found at 

 this horizon both in Vermont and Labrador. A species of 

 trilobite is referred to Olenellus, but I have not seen any illus- 

 tration of it. 



The conditions that developed the Middle Cambrian fauna 

 appear to have been largely peculiar to the American continent. 

 During the deposition of the St. John's series of the Lower 

 Cambrian, or the Paradoxides strata, we learn from the Euro- 

 pean and Eastern American sections, that the fauna was essen- 

 tially of the same type over the entire basin (Atlantic), and, 

 from evidence known to date^ that the fauna did not extend 

 west of a line passing northeast through Eastern Massachusetts 

 to New Brunswick and Newfoundland. 



That there were deposits of sediment to preserve the fauna, 

 if it extended westward, is shown by the thousands of feet of 

 sediments below the Middle Cambrian faunas of Utah and 

 Nevada. 



From the data we now have, I think that during the exist- 



