DEVONIAN FISHES OF IOWA 209 



Yet another view of Dipnoan interrelationships, or perhaps 

 rather to be considered as a modification of the first, is that 

 already outlined in the foregoing discussion of Arthrodires. The 

 organization of the existing Neoceratodus is regarded as decid- 

 edly more primitive than that represented by Dipterus and its 

 allies ; hence the modern structural type or its immediate proto- 

 type is assumed to have been in existence at least as early as 

 the lowermost Devonian, to have given rise to the more highly 

 specialized orders of Ctenodipterini and Arthrodira, and to 

 have persisted practically unchanged ever since. The modern 

 genus therefore falls within the same category as scorpions, 

 king-crabs, Lingula, Cestracion, Polypterus, Sphenodon and 

 other archaic survivals of far-distant faunas which have 

 manifested extraordinary persistence and conservatism through- 

 out the march of untold geological ages. 



This view may be said to rest almost entirely upon the evi- 

 dence of comparative anatomy, and has received as yet no 

 confirmation from the discovery of actual remains which fulfil 

 the requirements of a common ancestor to the three recognized 

 orders of Lung-fishes — Arthrodires, Ctenodipterines and Sir- 

 enoids. It may be that confirmation of this sort will never be 

 forthcoming, owing to the imperfection of the pakeontological 

 record. Nevertheless, relying upon the infallible clue of struc- 

 tural resemblances, one may project the divergent lines of 

 descent backward until they meet in a common point; and at 

 this point is to be sought the ancestry of the three orders with 

 whose history palaeontology acquaints us. 



The following passage, which occurs at page 500 of the same memoir, may be 

 contrasted with the views expressed by Bashford Dean in Science for July 12, 1907 

 (26, pp. 46-50), and February 7, 1908 (2 7, p. 203): 



Nachdem in jeder Beziehung Ceratodus als der primitivere erkannt ist, muss 

 auch die Annahme Dollo's, dass die Heterocerkie bei Dipterus etwas Primitives 

 bedeute, fallen. Der Schwanz von Ceratodus zeigt zwar gewiss Riickbildungen; 

 fur eine ehemalige Heterocerkie bei ihm ist aber kein Beweis erbracht. Ich kann 

 nach alledem, falls iiberhaupt eine Verwandtschaft zwischen Dipterus und den 

 recenten Dipnoern besteht, diese im Gegensatz zu Dollo nur im Sinne von Wood- 

 ward und Bridge auffassen. 



14 



