The Dipneusti and Crossopterygii 293 



CHAPTER X. 



The Evolution of Fishes. 

 II. The Dipneusti and Crossopterygii. 



The Dipneusti as a group is, and has been, probably the 

 most sohd, uniform, and least variable one in the entire 

 range of fish-life. For from first appearance in Devonian 

 rocks, up to the present day when three representative 

 genera exist in widely apart regions of the world, leading 

 types of the group have shown a unity of characters which 

 is in marked contrast with what is seen In other groups. 

 Accordingly Kerr (^j;^^) has well emphasized the num- 

 erous primitive characters observed in living genera. 



But the few now surviving seem to be a lingering rem- 

 nant of what was during Devonian or Old Red time, a 

 large, widely distributed, and abundant group, that has 

 left great accumulations of remains, either as entire animals, 

 as bony cephalic or trunk plates, or as teeth with or without 

 the dental bones. Unfortunately, of the soft parts we know 

 practically nothing in the fossil forms, though from the 

 standpoint of comparative morphology, such would have 

 shed light on many important questions. 



As we hope to prove in the subsequent context, all evi- 

 dently developed in extensive freshwater areas, that must 

 have been more or less in continuous relation, so as to permit 

 gradual distribution of the evolving species from Russia 

 and northern North America of the Atlantis continent, to 

 Africa, India and Australia of the Gondwana continent, 

 as well as to South America, by the time of the late Permian 

 or early Triassic. 



But in considering the evolution and gradual distribu- 

 tion of the entire group in widest relation, some recent 

 leading ichthyologists, and notably Eastman, have given 

 a much wider signficance to It, than was earlier done. For 

 Eastman brings together weighty arguments in favor of 

 uniting, with the more typical and long-lived group of the 

 Dipneusti, that of the Arthrodira, which are almost wholly 

 confined to Devonian rocks. He has developed his views 



