THE MISSISSIPPIAN PERIOD 617 



the counter-migration of the Osage fauna into the Great Basin 

 region; but this has not been determined. 



This persistence of Devonian types through to the end of the 

 Mississippian period, taken with the close continuity of the life of 

 the last Devonian epoch with that of the first Mississippian epoch, 

 and the absence of any notable physical break at that point, raises 

 the question whether the Mississippian might not better have been 

 regarded as the later portion of the Devonian period. This would 

 have given to the united period a cosmopolitan climax in the life 

 of the Osage-St. Louis limestones, and a fitting close in the decline 

 of many forms, and the unconformity at the top of the Mississippian. 

 The connection of the Mississippian with the Devonian, whether 

 physically or faunally, is closer than its connection with the Penn- 

 sylvanian. But no divisions of a history, which is in reality con- 

 tinuous, can be altogether without infelicities. The pulsations of 

 the history, which alone are the true basis of natural divisions, are 

 rarely the same everywhere at the same time. 



With the close of the Mississippian period, the chief center of 

 life interest passes from the sea to the land, first to the vegetation 

 of the Coal period, and then to the land vertebrates. The history 

 of the marine invertebrates will hereafter be followed with less full- 

 ness. With the introduction of fishes it had reached its great 

 adjustments, and its further history bears a close likeness to the 

 struggles and adaptations of the history already sketched. 



The Evolution of the Fishes in the Mississippian Period 1 

 Many of the ancient invertebrates were as fixed as plants, and 

 their migrations were confined to their early stages; but the fishes 

 were constant rovers of free and rapid movement. While restrained 

 by conditions of food, temperature, etc., they were relatively in- 

 dependent of local conditions. They appear to have effectually 

 invaded the open sea for the first time in the Devonian period, but 

 at that time true marine fishes seem to have been inferior, in number 

 and variety, to those of the inland waters. But by the middle of 

 the Mississippian period, the marine fishes had made such relative 



1 References: Newberry, Paleontology of Ohio, Vols. I and II; Woodward, 

 Vertebrate Paleontology; Dean, Fossil Fishes. 



