During the Cretaceous Period 



211 



freshwater and later anadromous types of animal might in- 

 creasingly multiply after they became marine dwellers, as 

 well as other important biological and ecological factors. 

 And one of the prominent results was the continued and 

 increasingly abundant migration of elasmobranch fishes, and 

 now even more of evolving teleost fishes, from a previous 

 freshwater to an acquired marine environment. 



Already we have seen that during the Jurassic period 

 various selachian, cestraciont, pycnodont, and pachycormid 

 fishes had branched off from freshwater ancestry into 

 marine life. But during the Cretaceous period, derivative 

 types from the freshwater "ganoids" gradually became 

 modified, in a manner that is traced in subsequent chapters, 

 along two lines of environal adaptation. One very large 

 and important series remained amid freshwaters, but rapid- 

 ly evolved into teleosteans of varied affinities and increas- 

 ingly perfected structure. Another, and at first less numer- 

 ous series, evolving also into teleostean types, passed into 

 seashores, later into the deep sea, and ultimately into 

 oceans and ocean depths. Exposed there to diverse en- 

 vironal conditions they evolved, through proenvironal and 

 selective reaction, modifications that, at the present day, 

 have culminated in forms often of grotesque, weird or de- 

 fensive aspect and structure. These are considered in de- 

 tail in chapters XII and XIII. 



