420 DEVONIAN PERIOD 



which the Southern Hamilton fauna lived (Fig. 358), but the inter- 

 vening barrier disappeared finally, and the northwestern fauna 

 overran the territory already occupied by the Southern Hamilton 

 fauna (Fig. 359). This northwestern fauna was closely allied to the 

 Devonian fauna of eastern and central Europe. The southward 

 extension of this great arm of the sea took place late in the period, 

 for the strata bearing its peculiar life lie on pre-Devonian formations 

 in Missouri, Iowa, and Minnesota, and overlie the Hamilton in the 

 more eastern region. 



Later Devonian (Chemung) fauna. The commingling and 

 conflict which attended the invasion of the eastern and southern 

 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 

 immigrants were the more virile, and gave character to the com- 

 posite fauna that arose later from the extinction of the weaker 

 species, and the adaptation of the survivors to one another. There 

 were three dominant factors in this development, (i) 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 showed itself in a 

 notable rarity of corals. The brachiopods best express the outcome 

 of the commingling of resident and immigrant species. Among 

 them, as in the whole fauna, there was an indigenous set of species 

 developed from the preceding residents, and an exotic set derived 

 from the immigrants and bearing North-European characters. The 

 latter was the more conspicuous. Among the mollusks, however, 

 the case was the reverse, and the majority seem to have been de- 

 scendants of the resident bivalves. 



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

 region of the west, a large area seems to have been occupied con- 

 tinuously by the sea from about the beginning of 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. Its 

 fauna had a slow, continuous 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 

 species. It is inferred, therefore, that it was shut off from the 



