298 Evolution and Distribution of Fishes 



phical positions, are given by Eastman for Iowa (iq8- 

 275-290). 



The higher DipneustI or lung fishes have shown a 

 surprising organic continuity, and fundamental similarity 

 of structure, from Lower Old Red to present times. Like 

 the Arthrodira, they developed as a purely freshwater 

 series, and they have continued to occupy this medium, in 

 all their extensive migrations and modifications, through 

 the many millions of years of their history. Eastman has 

 compared his own and DoUo's views as to their phylogeny 

 and relationship (op. cit. p. 208). 



The oldest known genus, Dipterus^ is represented by 

 two species from the freshwater Lower Old Red rocks of 

 N. Scotland. A species (Fig.9c, p. 119) — D. Valenciennesii 

 — is often extremely abundant in fairly large or even perfect 

 specimens from the base to the top of the series. Other 

 species have been described from the Devonian of Russia. 

 But a striking difference is that the latter are represented 

 only by isolated teeth or dental plates. This suggests that 

 the Scottish specimens were suddenly killed and very quick- 

 ly dried, to be later covered by mud; or they were quickly 

 sealed up under water by some volcanic ash-deposit, while 

 those from Russia evidently underwent prolonged macera- 

 tion, and after the hard parts had separated from the de- 

 cayed mass, they settled in some lake or river bottom. 



Representatives of the genus must steadily have spread 

 abroad from the northern European centre of evolution, 

 and had reached the eastern American area in early Upper 

 Devonian times. For from the freshwater Chemung of 

 Pennsylvania, from the nearly or quite contemporaneous 

 Catskill of New York, and even from the Cedar Creek and 

 State Quarry beds of Illinois and Iowa, remains of species 

 of Dipterus have been secured. But like specimens from 

 Russia, they are only isolated hard parts. 



Meanwhile, alike in its ancestral region as in the West, 

 the genus was splitting up and evolving into others that 

 departed more or less from the primitive type. So arose 

 Phaneropleuron (Fig. 12, p. 125) in the Scottish Canadian 

 region of the Upper Devonian, and Ctenodus of the Upper 

 Devonian and Calciferous Sandstone of America and 



