THE DEVONIAN PERIOD. 453 



the territory already occupied by the southern Hamilton fauna 1 (see 

 map, Fig. 196). 



Bearing in mind that these suggestions relative to the origin and 

 invasion of the America interior by these great provincial faunas are 

 subject to correction by new data, they may be taken provisionally 

 to represent the leading faunal movements of the early and middle 

 Devonian in the interior and eastern portions of the continent. These 

 successive invasions may be likened to the barbarian invasions of 

 southern Europe in medieval times. As in the latter case, the precise 

 origin of the invading hosts is subject to doubt and can only be approxi- 

 mately inferred, but the later stages of the invasions and their conse- 

 quences are more or less clearly discernible, and constitute the most 

 declared features of this romantic period. 



In the Great Basin there seems to have been a very slow and con- 

 servative evolution as though the area was largely isolated, or at least 

 protected from such faunal inundations as those that overwhelmed 

 the eastern region. This region and that of the Pacific coast will 

 be treated later. 



The Helderberg Fauna. 



Assuming that the Helderbergian fauna was derived from the 

 Silurian fauna, it should have been differentiated from it in a less 

 degree than were the Oriskany, Onondaga, and Hamilton faunas at 

 the time of their respective invasions, for less time had passed previous 

 to its appearance in the interior. This is conspicuously true. The 

 fauna bears a strong Silurian facies, and, on account of this, has often 

 been classed as Silurian. It had, however, undergone so much change 

 from the parent fauna that, with but two exceptions, so far as now 

 known, the species were all new. These two exceptions were the 

 cosmopolitan species Leptama rhomboidalis and Atrypa reticularis, 

 species remarkable for their wide range and long life. Considered as 

 species, they were the Methuselahs of the Paleozoic times. 



In that portion of the originating tract that lies in the St. Lawrence 

 embayment, there are over 2000 feet of shales and shaly limestones, 

 forming a continuous series, in the lower part of which the fossils are 

 predominantly Silurian and in the upper part predominantly Devonian, 

 while in the central thousand feet the two types are commingled or 

 1 Weller, Journ. of Geol., Vol. VI, p. 306. 



