PRINCIPLES OF PALEONTOLOGIC CORRELATION 68 1 



deposits known at that time could be grouped in well-defined 

 geographic provinces ; it is true that there was a great similarity 

 of animal life in the various regions, but this by no means 

 amounted to identity. Most genera had a wider range then 

 than in later formations, but community of species was as rare 

 in the Cambrian as in the later Mesozoic. Walcott has divided 

 the Cambrian into three great divisions, each named after the 

 most characteristic genus in it : the lower, or Olenellus zone ; 

 the middle, or Paradoxides zone ; and the upper, or Olefins zone. 

 No one of these had a universal fauna, although certain sub- 

 divisions can be traced through several provinces and even 

 regions, and thus deserve the designation "zone," as Oppel 

 used the word. It is noteworthy, too, that these times of inter- 

 regional distribution come at periods of transgression of the seas 

 on the land, so that a connection between these two phenomena 

 may justly be inferred. 



The work of Barrande, James Hall, and Murchison has 

 shown that the Silurian strata and their fossils are as widely dis- 

 tributed as the Cambrian. But during Lower Silurian time the 

 faunas of the American and the European region seem to have 

 been largely endemic. The Trenton sea probably covered the 

 greater part of North America, but only that in the northeastern 

 part of the region shows much relationship to the European. 

 During the Upper Silurian there was considerable readjustment 

 and shrinkage of the sea, and as a consequence of this the 

 Niagara limestone may justly be considered as an interregional 

 zone, although the exact period of the migration cannot be 

 determined. 



During the Lower and Middle Devonian the division into 

 regions that had existed in the Silurian still held sway, for it has 

 been shown by H. S. Williams that there was a North-South 

 American and a Eurasian region. But with the beginning of 

 the Upper Devonian the connections had changed so that the 

 grouping was Eurasian-North American, and South American- 

 South African. This change shows itself in North America at 

 the base of the Upper Devonian, where with the Cnboides zone 



