62 IOWA GEOLOGICAL SURVEY 



At the same time that early Elasmobranchs were strength- 

 ening and otherwise improving their paired fins, similar modifi- 

 cations were in progress among two other groups of typical 

 fishes which suddenly became dominant during the Devonian. 

 One of these groups is that commonly known as Dipnoans 

 (Dipnoi, or more properly, Dipneusti), the name of "double- 

 breathers" referring to their power of respiration by both gills 

 and lungs. The second competing rival in the line of advance is 

 the group of Crossopterygii, or "fringe-finned ganoids". Both 

 are possibly descended from primitive Elasmobranchs, and both 

 are conspicuous for their conversion of paired fins into paddles 

 suitable for crawling in the mud, or for ordinary swimming in 

 water. A curious feature of evolutionary progress is that to 

 which Smith Woodward has called attention in another recent 

 article,* namely, no sooner had fishes acquired the paddle-shaped 

 paired fins than "they suddenly became the special feature of 

 the Devonian period in all parts of the globe that have hitherto 

 been geologically examined, and they attained their maximum 

 development, being more numerous and more diverse in form 

 than at any subsequent time." 



The paddle-shaped type of fin became prevalent among the 

 highest fishes at about the very epoch (Middle and Upper De- 

 vonian) when terrestrial four-legged vertebrates were just be- 

 ginning to make their appearance. The coincidence is note- 

 worthy, and favors the current opinion that Labyrinthodont 

 amphibians (Stegocephalia) are descended from primitive 

 Crossopterygii. There are still some controverted points in 

 regard to this theory, whose consequences are far-reaching, hence 

 it will be instructive to compare the views of the English author 

 we have already quoted both on this matter and on the devel- 

 opment of effective fins in higher fishes. His diction is given in 

 slightly condensed form in the following paragraph. 



"During the Middle and Upper Devonian there was a general 

 tendency for the most advanced fishes to become crawlers rather 

 than swimmers ; and there cannot be much doubt that the known 

 Crossopterygii are the unsuccessful survivors of the race which 



*Woodward, A. S., The Relations of Palaeontology to Biology. Ann. Mag. Nat. 

 Hist. 1906, ser. 7, 18, p. 315. Extract from an address delivered before the Inter- 

 national Congress of Arts and Science, St. Louis. 



