During the Cretaceous Period 217 



A significant feature of the marine beds with their in- 

 vertebrate organisms, and evolving marine fishes, is the 

 total absence of amphibians which have always and wholly 

 frequented freshwater or the land. Thus if one overhaul 

 the classified list prepared by the Committee of the British 

 Association {168: 149) or similar more recent lists, one 

 notices that while various amphibian genera are listed from 

 Carboniferous, Permian, Triassic, Keuper, Bunter and 

 other rocks of freshwater origin, none are found mixed 

 with marine types. 



In marked contrast to this was the formidable invasion 

 of the sea by saurians, that were undoubtedly land dwellers 

 in their earlier origin and history, but which from Jurassic 

 and onward to early Cretaceous days passed seaward, and 

 underwent striking modifications there. ^ Most of the 

 Plesiosauria and Pterosauria, so far as evidence shows, re- 

 mained from Triassic to Cretaceous time as land, air or 

 lake dwellers, and then died out. But many had a formid- 

 able organization for preying on other animals. These 

 and succeeding derivative marine reptiles preyed heavily 

 on fishes, as their coprolites demonstrate. So not a few 

 of the fishes per force took to a marine life. And this 

 biological relation it is, which we would with all caution 

 advance, as the main explanation for the migration seaward 

 of many groups of fishes, during late Jurassic and early 

 Cretaceous times. Then such a genus as Elasmosaurus 

 amongst Plesiosaurians, which in some of its species largely 

 remained on land or in freshwater, also the entire group 

 of the Ichthyosauria, and of the Pythonomorpha in their 

 later history, followed the fishes seaward, and became 

 remarkably modified accordingly. The formidable rows of 

 teeth that they developed, the often elongated snout along 

 which these were disposed, the spiral valve of the intestine, 

 the crushed remains of cephalopods and of fishes often 

 found fossilized in their digestive tract, and the complete 

 modification as well as condensation of the limbs after the 

 pattern of a cetacean, all demonstrate that they were form- 

 idable sea animals and could pass to a great distance from 

 land in search of prey. 



