6oo The Crossopterygii 



more nearly related to the Ganoids, and through them to the 

 ordinary fishes. 



Origin of Amphibians. From the primitive Crossopterygii 

 the step to the ancestral Amphibia, which are likewise mailed 

 and semi-aquatic, seems a very short one. It is true that most 

 writers until recently have regarded certain Dipneustans as 

 the Dipteridcs as representing the parents of the Amphibians. 

 But the weight of recent authority, Gill, Pollard, Boulenger, 

 Dollo, and others, seems to place the point of separation of the 

 higher vertebrates with the Crossopterygians, and to regard 

 the lobate pectoral member of Polypterus as a possible source of 

 the five-fingered arm of the frog. This view is still, however, ex- 

 tremely hypothetical and there is still much to be said in favor 

 of the theory of the origin of Amphibia from Dipnoans and in 



FIG. 370. Shoulder-girdle of Polypterus bichir. Specimen from the White Nile. 



favor of the view that the Dipnoans are also ancestors of the 

 Crossopterygians. 



In the true Amphibians the lungs are better developed 

 than in the Crossopterygian or Dipnoan, although the lungs are 

 finally lost in certain salamanders which breathe through epithe- 

 lial cells. The gills lose, among the Amphibia, their primitive 

 importance, although in Proteus anguineus of Austria and 

 Necturus maculosus, the American "mud-puppy" or water-dog, 

 these persist through life. The archipterygium, or primitive 

 fin, gives place to the chiropterygium, or fingered arm. In 



