196 THE ORIGIN AND EVOLUTION OF LIFE 



tilian sea fauna of plesiosaurs and mosasaurs of Upper Creta- 

 ceous time. 



In the meanwhile the Ufe of the continents is revealed in 

 the terrestrial and fluviatile deposits of the Triassic Epoch, 

 in the first stages of the terrestrial evolution of the dinosaurs, 

 in the early stages of the fluviatile evolution of the Crocodilia, 

 and in the final stages of the terrestrial phases of the Amphibia 

 and pro-Reptilia. A long interval of time elapses at this 

 period in the earth's history, during which the life of the con- 

 tinents is entirely unknown, until the close of the Jurassic 

 and beginning of Cretaceous time, when there appears a sec- 

 ond great stage of dinosaur evolution, revealed especially in 

 the lagoon deposits of North Africa and South America, which 

 have yielded remains of giant Sauropoda. Then another gap 

 occurs in the story as told by continental deposits. Finally, in 

 Upper Cretaceous time we again discover great flood-plain and 

 shore-line deposits, which give a prolonged vista of the ter- 

 restrial life of the Reptilia, especially in North America and 

 Europe. 



Thus it will be understood that, while the great tree of 

 reptilian descent has been worked out through a century of 

 scientific researches, beginning with those of Cuvier and con- 

 tinued by Owen, Leidy, Cope, Marsh, and our contemporary 

 palaeontologists, there are enormous gaps in both the terres- 

 trial and the marine history of several of the reptilian orders 

 which remain to be filled by future exploration. We piece to- 

 gether fossil history on the continents and in the seas from 

 the animals entombed in these deposits, partly by means 

 of the real relationships observed in widely migrating forms, 

 such as the land dinosaurs and the marine ichthyosaurs, ple- 

 siosaurs, and mosasaurs. Many of these reptiles ranged over 

 every continent and in every sea. On the whole, the physio- 



