THE DEVONIAN PERIOD. 477 



northern distribution of both. The corals of the northwestern Hamil- 

 ton fauna were more abundant than those of the southern Hamilton 

 fauna, but much inferior to those of the Onondaga. 



The echinoderms were a minor factor, as were also the trilobites 

 and bryozoans. The pelecypods were less abundant than those of the 

 southern Hamilton and different from them. Gastropods were pres- 

 ent, but not important, and the same may be said of .the cephalopods. 



Some notable fish remains have been found in Iowa, but they are 

 quite different from these of the eastern area, although perhaps derived 

 from them by westward counter-migration, after the communication 

 between the provinces was established. 



In the later part of the Hamilton epoch, when the southern and 

 northwestern seas became confluent, the northwestern fauna invaded 

 and overran the eastern and southern territory, and a new fauna 

 developed from the commingling of the members of the two. This 

 leads on to the closing, more cosmopolitan phase of Devonian life in 

 the interior sea, the Chemung fauna. 



The Later Devonian (Chemung) Fauna. 



The commingling and conflict which attended the invasion of 

 the eastern and southern interior sea by the northwestern fauna may 

 be regarded as the initial and controlling event in the evolution of 

 the Upper Devonian fauna. As in the case of the Onondaga invasion, 

 the northern fauna proved the more virile and gave character to the 

 composite fauna that resulted from the extinction of the weaker species 

 and the fusion of the remaining invaders and invaded. The strati- 

 graphical evidence of the invasion is first caught on the western border 

 of the invaded sea in the fauna of the Hamilton at Milwaukee. The 

 northwestern invaders appear first in the eastern part of the basin 

 in the Tully limestone of New York, where the detection of the sig- 

 nificance of the fauna is due to the critical studies of H. S. Williams. 



The influence of faunal and physical conditions. — From the fact 

 that the sediments of the time were predominantly silts and sands, 

 only a slight and precarious submersion of the occupied portions of 

 the continental platform is inferred, and this was not a condition 

 altogether favorable to the evolution of a general fauna of the first 

 order, and the local conditions were effectively impressed on the fauna. 

 In the interior basin there were three dominant factors: (1) the resi- 



