230 Evolution and Distribution of Fishes 



and Stewart (^^5:385), in which a comparison of the 

 various fish genera from two or more localities is made. 



But though the above details are impressive, they must 

 by no means prevent us from realizing that this entire 

 marine association of fishes represents derivative offshoots 

 from more primitive freshwater elasmobranchs, ganoids, 

 or teleosts. Further a very abundant freshwater fish fauna 

 co-existed with them. For we have ample evidence that a 

 few elasmobranchs, all of the Dipneustei, most of the 

 "ganoids," and a very large series of the more primitive 

 teleosts remained in freshwater habitats. These will be 

 studied in detail later, but here it need only be observed that 

 over a large part of North America, not to mention other 

 continental areas, after deposition of the marine Fox Hills 

 group, there succeeded the Montana, Laramie and Living- 

 stone that represent several thousand feet of strata laid 

 down mainly in great inland lakes. The land surrounding 

 these was in large measure covered by a very rich flora in 

 part of gymnospermic, in part of angiospermic character. 

 An equally abundant insect, molluscan, and vertebrate fauna 

 occurred on the land or teemed in the lakes. Amongst the 

 vertebrates were many ganoid and teleostean fishes, turtles, 

 crocodiles and other giant reptiles. 



Widespread volcanic activity, with corresponding de- 

 struction of plant and animal life, took place over widely 

 apart regions of the earth during later Cretaceous time. 

 This along with the steady evolution of predaceous and 

 ferocious vertebrates, also the steady spread of plant and 

 animal parasites that must often, as now, have caused havoc 

 to higher forms explains the great blotting out of life, as 

 passage is made from upper Cretaceous to lower Eocene 

 beds. Striking verification of this Is given later (p. 231). 

 But the uniformity and continuity of bedding seen in some 

 regions indicate that similar biological continuity was pos- 

 sible, though doubtless this was accompanied by steady 

 variation and new envlronal adaptations. 



