5i6 Evolution and Distribution of Fishes 



group in the entire range of fish life. Such is in line with 

 Kerr's emphasis on "the numerous primitive characters 

 observed in living genera." All evidently evolved in, and 

 have remained till now in freshwater areas. As in the case 

 of other groups already reviewed, the earlier or Palaeozoic 

 forms spread by degrees from their probable ancestral evo- 

 lutionary centre in the east part of the Eria or Atlantis 

 continent, and from the Old Red period onward till they 

 had reached Iowa in the west, also by some Atlanto-Gond- 

 wana bridge to Africa, India, and Australia. If moreover 

 East's view be accepted that the scaleless arthrodires are 

 Dipneusteans — as seems likely — then also if the oldest 

 known fragments of Coccosteus are correctly diagnosed, it 

 follows that these existed during the late Silurian period. 

 Similarly if the view be accepted that the scaled types or 

 Dipnoi evolved from more primitive organisms that com- 

 bined characters of the Silurian Birkenia with those of 

 primitive Pleuracanthidae, these also would date back to 

 the Silurian. 



The earlier and smaller arthrodires like Coccosteus, of 

 Lower Old Red strata, seem when traced from Europe to 

 America and when studied there in ascending strata up to 

 the topmost Old Red beds, to increase in size and to assume 

 the characters of the giant Dinichthys of Ohio and other 

 central states. The abundance of these and contemporary 

 fish-groups in Upper Old Red rocks seem to have originated 

 the petroleum supplies of the Cleveland Shale and related 

 beds. Eastman's valuable lists of fishes for New York, 

 Iowa, and elsewhere are referred to. The scaled Dipneu- 

 sti or Dipnoi (lung fishes of modern times) are abundant 

 Lower Old Red fossils. Dipterus as a primitive genus 

 spread probably from Central Atlantis or Eria, till the 

 area from Russia to Central N. America was invaded in its 

 rivers and lakes. More recent genera like Ctenodiis and 

 Sagenodus of Carboniferous age, connected with Gosfordia 

 of Permo-Triassic lake life; while the world range was 

 extended by degrees till Australia at least was reached. 



Ceratodtis again continued the group biologically from 

 Triassic upward to Cretaceous days, while the very close 

 resemblance of it to the living Neoceratodus of E. Aus- 



