THE DEVONIAN PERIOD. 469 



to have gone so far at length as to close the eastern straits through 

 which the Helderberg and Oriskany faunas had entered. In the 

 southwestern region the w r arping was downward and permitted the 

 sea to creep out upon the previous land, as shown by the overlap of 

 the Hamilton formation. There is no positive stratigraphic proof 

 that this went so far as to make a connection with the ocean in that 

 direction, but this seems highly probable. It is quite obvious from 

 the nature of the life that an oceanic connection was maintained some- 

 where, and there is an absence of evidence that it was maintained 

 either to the northeast or to the north, for no Hamilton beds are found 

 in those directions, nor are there any faunal indications of such a 

 connection. On the other hand, the fauna contains peculiar forms 

 found also in South America, and this fact, together with certain fea- 

 tures of distribution within the interior, strengthens the presumption 

 of a connection lying somewhere in the lower Mississippi tract. Unfor- 

 tunately this region is very deeply buried beneath younger deposits, 

 and a stratigraphic demonstration of the continuity of the Hamilton 

 sediments through the supposed strait is impracticable. 



Assuming that these were the physical conditions which led to 

 its introduction, the earlier Hamilton fauna may be regarded as the 

 result of a resident clear-water fauna contending with increasing tur- 

 bidity of water on the one hand, and with the invasion of a southern 

 fauna on the other. It involved the domestic modification of the 

 resident fauna to fit new conditions, and the absorption and accom- 

 modation of foreign accessions, for the latter do not seem to have 

 overpowered the former. It was not therefore so radical a transforma- 

 tion as that which attended the introduction of the Onondaga fauna, 

 when the invaders were not only the master type, but were aided by a 

 progressive physical change in their favor. 



The progress of the sharks and arthrodirans. — The relics of Hamil- 

 ton fish thus far recovered are less numerous than those of the pre- 

 vious epoch, and, like them, are largely fragmentary, but they indi- 

 cate an advance in structural development. The arthrodirans reached 

 their climax in massiveness and distinctive characters. The Dinichthys 

 herzeri, the leading type, was one of the largest and most formidable 

 fish known, with an estimated length of 20 feet, and very massive pro- 

 portionately. It was armed with formidable mandibles two feet in 

 length, which, in lieu of teeth, were developed into shear-like and beaked 



