Summary of Conclusions Reached 511 



that a great and varied extension of elasmobranchs into 

 the sea now took place. It also emphasizes the view that the 

 Dipnoans, most of the Holosteans, the Chondrosteans, and 

 most of the 'ganoids,' as well as many of the derivative 

 Teleosteans remained as freshwater dwellers. Here also 

 it may be observed that the only genera of the fishes which 

 occur in Jurassic strata and have come down through the 

 Cretaceous to our own day in living representatives are 

 Rhina, Rhinobatus, Notidanus, Cestracion, Pristiurus, and 

 Ceratodus," all of which had apparently become thoroughly 

 marine by the beginning of the Cretaceous, except Cera- 

 todus which persisted throughout in freshwater. Frequent 

 continuity stratigraphically with Purbeck beds is accepted, 

 and a parallelism is drawn between Cretaceous beds of the 

 Old and New Worlds. The tremendous volcanic activity 

 then proceeding is evidenced by the Deccan traps of India, 

 and the Laramide beds of the Western States. Such 

 activity must have affected the whole earth. As a partial 

 result, commencing upheaval into conspicuous mountain 

 chains, and depression into ocean depths now started. Such 

 favored rapid evolutionary changes. The commencing 

 Jurassic migration of various selachian, cestraciont, pycno- 

 dont and pachycormid fishes into marine surroundings was 

 now added to, owing to like migration of teleost derivatives 

 from more primitive ganoids. 



The nature of the rocks, and the enclosed organisms 

 found in Wealden and in successively higher strata of the 

 system indicate that derivative marine fishes — mainly elas- 

 mobranch or teleost — were gradually becoming more nu- 

 merous than freshwater types, and so occur now abundantly 

 alongside typical marine invertebrates. Further, while 

 amphibians are wholly unrepresented in the latter beds, 

 giant saurians that had passed seaward during Jurassic 

 time are common, and had largely aided probably in driv- 

 ing groups of fishes seaward also. While dipnoan and 

 ganoid fishes still abound in freshwaters, a small and steadi- 

 ly specializing ganoid derivative line passed seaward as 

 the marine pycnodont fishes, and these so persisted till their 

 extinction in Upper Eocene time. With them were deriva- 

 tive teleost forms "from the Eugnathidae, Pachycormidae, 



