During Triassic and Jurassic Periods 189 



crushing teeth, that made them formidable antagonists. 

 Fourth: the body-covering of scales, the body-muscles, and 

 also the fins became of a highly perfected type In these. 

 Fifth: from having had few antagonists equal to or more 

 powerful than themselves in freshwater, as the Increasingly 

 huge carnivorous reptiles evolved, these evidently pressed 

 on the freshwater fishes, drove them to sea, and even then 

 many species and genera of reptiles — following the fish- 

 shoals — became modified as sea-dwellers. Sixth: the 

 abundant invertebrate life in the sea, that would serve as 

 suitable food, aided the persistence, multiplication and 

 •dissemination there of elasmobranchs. Seventh: consider- 

 able parts of subaerial areas, during the Permian and the 

 Triassic, became of a dry xerophytic and seml-desertic 

 nature, while simultaneously subject to considerable denuda- 

 tion, and to formation of wide lacustrine expanses that 

 often became shallow and saline. So not a few previously 

 freshwater organisms, — Including fishes, must have become 

 accustomed to a fairly saline environment. Preparation 

 was thus made for passage toward the sea of those types 

 which best survived in the saline environment. Eighth: the 

 climax of cephalopod development was reached during low- 

 er Jurassic time and then decreased. Possibly also medusoids, 

 related to those so beautifully preserved in the Solenhofen 

 slates may up to this time have been abundant and trouble- 

 some marine dwellers, but now may have begun to dwindle 

 In importance. Opportunity was thus at length given for 

 passage of the most predaceous or otherwise adapted, of 

 the freshwater fishes into the sea. The Selachians first, and 

 later lateral derivatives from the "ganoids" seized the 

 opportunity. 



As emphasized by Sauvage {143: i) and other geolo- 

 gists and palaeontologists, the Rhaetic or Upper Triassic 

 beds pass almost insensibly Into those of the Liassic, while 

 we have already incidentally indicated that not a few plant 

 and animal genera are continuous through both formations. 

 But while extensive changes In the relation of land and 

 sea seem to have proceeded slowly during Triassic times, 

 oscillations between land and sea seem to have been fre- 

 quent and variable during the Jurassic. So marine or 



