582 



GEOLOGY 



The Southern Hamilton fauna. At the beginning of the Ham- 

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

 eastern part of the interior sea, while the formation of limestone 

 continued as before in the western part. This appears to have 

 been the result of crustal warping, which affected stream-erosion 

 and inwash on the east, and closed the straits through which the 

 Helderberg and Oriskany faunas had entered, while in the south- 

 west, downward warping made a more open connection between 

 the interior sea and the ocean in that direction. At any rate, it 

 appears that a fauna whose relatives lived in South America, 

 entered the interior sea, and, joining the resident Onondaga fauna, 

 led to the evolution of the Southern Hamilton fauna. In the 



Fig. 417. Diagrammatic front view of the dentition of Dinichthys herzeri, 

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



earlier stages, the resident clear-water fauna was thus forced to 

 contend with the increasing turbidity of water on the one hand, 

 and with the southern immigrants on the other. This led to a 

 change of the resident fauna to fit the new conditions, and to the 

 absorption and accommodation of the invaders. It was not so 

 radical a transformation as that which attended the previous in- 

 vasion from the north that gave rise to the Onondaga fauna. 

 because then the invaders were the master type. 



The fishes played a conspicuous part in the new fauna. 1 The 

 arthrodirans reached their climax, and some of the species were 

 among the largest and most formidable fish ever known. Dinich- 

 thys (Fig. 417) had an estimated length of 20 feet, and was armed 

 with formidable mandibles 2 feet in length which, in -lieu of teeth, 



1 Eastman, Mem. N. Y. State Mus., Vol. X. 



