THE DEVONIAN PERIOD 587 



interior sea by the European and Eurasian faunas may be regarded 

 as the controlling event in the evolution of the Upper Devonian 

 fauna. As in the case of the Onondaga invasion, the northern fau- 

 na proved the more virile, and gave character to the composite 

 fauna that later arose from the extinction of the weaker species 

 and the adaptation of the survivors to one another. There were 

 three dominant factors in this development, (1) the resident Southern 

 Hamilton species, (2) the invading European and Eurasian species, 

 and (3) the shallow and rather turbid waters in which these species 

 met and merged. The last of these factors expressed itself in a 

 notable rarity of corals. Though the turbid waters would hardly 

 have been congenial to the crinoids, they were represented, as well 

 as a few other echinoderms. The brachiopods best express the 

 outcome of the commingling of resident and immigrant species. 

 As in the case of the total fauna, there was an indigenous set of 

 species developed from the preceding residents, and an exotic set 

 derived from the emigrants and bearing North-European char- 

 acters. The foreign group was more conspicuous than the native. 

 Among the mollusks, however, the case was the reverse, and the 

 majority seem to have been descendants of the resident bivalves. 

 The record of the fishes was a decline in the arthrodirans, an increase 

 in the sharks, and a continuation of_the ganoids without great change. 

 The record of the minor forms was not especially characteristic. 



The Devonian fauna in the Great Basin area. In the region 

 which constitutes the Great Basin of the west, a large area seems 

 to have been occupied continuously by the sea from about the be- 

 ginning of the Middle Devonian time to the later portion of the 

 Carboniferous period. It seems to have been measurably free 

 from both the physical and the biological changes which gave such 

 diversity to the eastern provinces. The fauna had a slow, con- 

 tinuous evolution, favored, from time to time, it would appear, by 

 accessions from the north and perhaps from other sources as well. 

 None of the distinctive South American forms appeared in it, nor 

 any of the peculiar Helderberg or Oriskany forms. It is inferred, 

 therefore, that it was disconnected from the eastern and southern 

 interior throughout the whole Devonian period. On the other 

 hand, a notable number of species were common to it and to the 



