THE DEVONIAN PERIOD 575 



ing faunas have been recognized: (1) the Helderberg, (2) the 

 Oriskany, (3) the Onondaga (Corniferous), (4) the Southern Ham- 

 ilton, and (5) the Northwestern Hamilton fauna. They reached 

 the interior in the order named. As each in turn came in contact 

 with the previous fauna, there was a commingling and conflict of 

 the two, resulting in the destruction of some species and the adjust- 

 ment of others to one another. The final result was the develop- 

 ment of a new, composite fauna from the survivors. 



The Helderberg fauna. The Helderberg fauna seems to have 

 developed from the late Silurian fauna in the embayment at the 

 mouth of the St. Lawrence and on the border of the adjacent con- 

 tinental shelf, and perhaps also on the border of southern Europe. 

 The fauna appears to have found a way into the Appalachian 

 valley-trough and thence to have spread westward and northward 

 as far as the advancing waters of the time permitted. Perhaps 

 there was access to the interior also from embayments on the 

 southern coast. The fauna seems never to have occupied more 

 than the eastern part of the great interior region. The fauna of 

 similar date in southern Europe (Hercynian) had much in common 

 with the Helderberg fauna of America, but both differed markedly 

 from the earlier Devonian faunas of the more northern latitudes 

 of Europe and America. 



The main features of the Helderberg fauna were a marked 

 development of the mollusks, great numbers of the molluscoids, an 

 erratic tendency of the trilobites, a scantiness of crinoids and corals, 

 and a notable absence of fishes. Some of these features were 

 doubtless due to the physical conditions of the originating tract 

 where there was a muddy, partly calcareous bottom. So far as 

 like conditions prevailed in the interior to which the fauna emi- 

 grated, its original characters were retained, as shown by the abun- 

 dance of mollusks and molluscoids, and the fewness of corals and 

 crinoids. The gastropods (p. Fig. 414) came into prominence, espe- 

 cially in the rather inferior capulid type. Certain pelecypods of the 

 winged type (aviculids, o, Fig. 414) were also conspicuous. The 

 brachiopods surpassed all other classes in numbers but did not 

 develop such distinct peculiarities as the gastropods and pelecy- 

 pods. The trilobites showed a tendency to sport, taking on strange 



