4i8 



DEVONIAN PERIOD 



Southern Hamilton fauna. At the beginning of the Hamilton 

 epoch, there was a great influx of muddy material into the eastern 

 part of the interior sea, while farther west the formation of limestone 

 continued as before. At about this time, it appears that a fauna 

 whose forbears lived in South America entered the interior sea, and, 

 joining the resident Onondaga fauna, gave origin to the Southern 

 Hamilton fauna. The transformation was not so radical as that 

 which attended the invasion which gave rise to the Onondaga fauna, 

 because the invaders were then the master type. 



Fishes were a conspicuous part of the new fauna. The arthro- 

 dirans reached their climax, and some of the species were among 

 the largest fish ever known. Some of them had an estimated length 

 of 20 feet, and had strong mandibles 2 feet long (Fig. 366) which, 



Fig. 366. Diagrammatic front view of the dentition of Dinichthys hcrzeri, 

 Huron Shales, Delaware, O. (After Newberry.) 



in lieu of teeth, had cutting edges that closed, shears-like, after the 

 fashion of the mandibles of turtles. The front part of the body was 

 encased in heavy plates. Some of the fin-spines of sharks were a 

 foot long. In both groups of fish the devices of warfare make up 

 nearly the whole record, and this doubtless implies the conditions 

 in which the vertebrates lived. 



Polyps were affected adversely by the muddy waters. Crinoids 

 were abundant locally, certain beds of limestone being composed 

 largely of their remains. Brachiopods reached their climax at 

 about this time. Among them, the spirifers attained their greatest 

 extension of hinge-line (j, Fig. 367) a feature characteristic of the 

 Hamilton epoch. The muddy bottoms favored mollusks. Gonia- 

 tites increased in numbers and size (Fig. 367, 0), and pelecypods 

 still more, the number of known species approaching 200. At this 

 time appeared the first known barnacles of the modern sessile type. 

 In losing its pedicel and in fixing itself immovably on other objects, 

 it became degenerate, but it found a lowly place to which it has 



