The Dipneusti and Crossopterygii 301 



greater. Further of the three species now alive Boulenger 

 has shown (227:37-42) that P. dolloi which extends over 

 the lower reaches of the Congo in Africa, shows nearest 

 affinity to Lepidosiren, which is a highly modified type 

 of the group. Such suggests that Lepidosiren was possibly 

 not derived from migration southward of a North Ameri- 

 can type, but is the specialized end-member of a series 

 that migrated westward from an ancient Australian habitat. 



Ceratodus seems to have lingered on in North America 

 till the close of the Cretaceous or early Eocene epoch. For 

 from the Laramie beds, that have been variously interpreted 

 as Upper Cretaceous or Lower Eocene, Cope has described 

 some rather imperfect tooth-plates under two specific 

 names. Then from New South Wales, teeth that were 

 found in Tertiary alluvial beds indicate that while the 

 more modified Neoceratodus was evolving from the older 

 genus in Queensland, Ceratodus itself was nearing ex- 

 tinction. 



In review then it appears that the arthrodire-dipnoan 

 phylum of fishes evolved in freshwaters of Old Red or even 

 of late Silurian age, and that the focus of evolution was 

 located in lakes or rivers of North-Central Europe. Thence 

 a more or less continuous extension westward to Illinols- 

 lowa, and southward to Texas, took place before latest 

 Devonian time. Then there had evolved those clumsy 

 forms the sub-group of arthrodires in which the scales be- 

 came absorbed — as has often happened amongst tribes of 

 higher fishes. These ranged from Europe to Central North 

 America, but attained a climax of defensive specialization 

 though ungainly awkwardness in Dinichthys and Titanich- 

 th\s of the Upper Devonian in North America. But the 

 smaller and more active scaled dipnoans reached, probably 

 by a Turkestan-Indian line of passage, the eastern or 

 Australian part of Gondwana by the close of devonian days, 

 and there permanently persisted in descendant represen- 

 tatives. Outliers of the latter floated westward at a later 

 period into African Gondwana, over the larger part of 

 which they are now spread. From the most specialized of 

 the three African species, namely P. dolloi of the Lower 

 Congo, still later outliers passed westward across the great 



