226 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 Dipteride 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 
Fia. 159. —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 
