438 



Comparative Morphology of Chordates 



tion of the skeleton they resemble Chondrostei. The scales of existing 

 Dipnoi are similar to those of Teleostei. The skeleton of their paired 

 fins is quite unlike that of any known living fish. Also their geographic 

 distribution — a single genus in each of three widely separated land 

 areas — is perplexing. The several questions to which the modern 

 dipnoans give rise are answered to some extent by paleontology. Evi- 



Fig. 338. Pelvic fin and part 

 of girdle of Epiceratodus. 

 (ax) Axial skeleton of fin; 

 (pil) iliac process; (pirn) 

 processus impar; (r) radialia. 

 (After Davidoff. Courtesy, 

 Kingsley: "Comparative 

 Anatomy of Vertebrates," 

 Philadelphia, The Blakiston 

 Company.) 



dence from fossils indicates that fishes of the dipnoan type have existed 

 continuously since very early geologic times, being traceable back to 

 nearly the middle (Devonian Period) of the Paleozoic Era. In ancient 

 times these fishes were much more numerous and more widely dis- 

 tributed than now. The fossil skeletons of the older dipnoans have 

 many points of resemblance to those of the crossopterygians con- 

 temporary with them. These crossopterygian skulls show evidence of 

 internal nares (choanae) which would imply air-breathing and lungs — 

 although in rare cases, in the absence of lungs, the nasal cavity of a fish 

 may open, in one way or another, into the mouth-cavity, as in the 



