534 GEOLOGY. 



of the Osage fauna into the Great Basin region; but this does not 

 seem to have been worked out. With little doubt the conservative 

 invaders of the interior sea were favored by physical conditions, which 

 appear to have grown progressively more and more like those to which 

 the Devonian types had become adapted, and less and less like those 

 to which the special Osage forms were fitted. There is danger, how- 

 ever, of over-estimating the Devonian facies, because of its interest. 

 The total assemblage bore a pronouncedly Mississippian aspect. 



This persistence of Devonian types through to this, the last recog- 

 nized epoch 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 closing 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 summit of the Mississippian. 

 But no divisions of a history, which is in reality continuous, can be 

 altogether without infelicities. The pulsations of the history, which 

 alone are the true basis of natural divisions, are rarely the same every- 

 where at the same time, and in all their aspects. 



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 therefore be followed with less fullness. With 

 the introduction of fishes it had reached its great adjustments, and 

 its further history bears a close likeness to the struggles and adapta- 

 tions 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, quite in con- 

 trast, the fishes were constant rovers, and their distribution was rela- 

 tively free and rapid. While restrained by conditions of food, tem- 

 perature, etc., they were relatively independent of local conditions, and 



1 References: Newberry, Worthen and St. John, 111. Geol. Surv., Vols. II, IV, VI, 

 VII; Newberry, Pal. Ohio, Vols. I and II; Leidy, Trans. Am. Phil. Soc, Vol. XI; 

 A. S. Woodward, Vert. Pal.; Dean, Fossil Fishes. 



