i88 Evolution and Distribution of Fishes 



important. He says : "The fragmentary skeleton of Sageno- 

 diis is of great interest when considered in connection with 

 other recent discoveries of extinct dipnoan fishes in Austra- 

 lia. There is now evidence of fore-runners of the surviving 

 Ceratodtis in the Devonian of New South Wales {Ganor- 

 hynchtis sussmilchi) , the Carboniferous of Victoria {Cten- 

 odus breviceps) , the Permo-Carboniferous and Triassic of 

 New South Wales {Gosfordia truncata), and the Jurassic 

 of Victoria {Ceratodiis aviis) . It is thus clear that Dipnoi 

 have always lived in the Australian region, and there is no 

 reason why Ceratodiis may not have evolved there." It 

 must be remembered however that Ceratodiis was once 

 practically world-wide, and that the richest and oldest 

 known centre for dipnoans was the British-Russian area 

 of the Lower Old Red. But the remarkable distribution 

 of the perennially freshwater group Dipnoi is one of the 

 outstanding proofs that some fishes have tenaceously clung 

 to their primitive environment. 



The Jurassic Formation. 



The Jurassic Formation is of surpassing interest from 

 the standpoint of fish-life. For it is in this that we again 

 have clear evidence of migration from a freshwater to a 

 marine life. Several factors seem to have cooperated to- 

 ward such a result. First, geologists are agreed that a 

 considerable — in some places great encroachment of the 

 sea on the Triassic and Permian land-areas took place, 

 which may be said to have reached an apparent climax in 

 our own day. During this shifting process great masses 

 of fishes became exterminated, while others became modi- 

 fied and adapted to changed environment. Second: this 

 evidently inclined many hitherto freshwater or anadromous 

 animals to adopt a semi-marine and eventually marine life, 

 through struggle for existence in the former areas along- 

 side evolving and formidable reptiles. Third: From being 

 rather unwieldy animals with — in many cases — a dentition 

 that was poorly adapted for combat and offensive destruc- 

 tion, one group of fishes — the Elasmobranchii — evolved 

 highly specialized catching, tearing and often in addition 



