THE VERTEBRATA 229 



3. The Dipnoi are interesting as being the fishes in 

 which there is a transition to the internal structure of 

 air-breathing vertebrates : at least this is the case with 

 existing survivors of the group, but many extinct forms 

 are associated with them on account of their skeletal 

 characters. The Dipnoi appear as early as the ganoids, 

 and, like them, were both marine and freshwater, but 

 the three surviving genera are confined to rivers and to 

 the Southern Hemisphere (in curious contrast to the 

 ganoids). The best-known of these modern genera is 

 the Australian Ceratodus (Fig. 66, a), which has a pair 

 of large ridged palatal teeth, formed by the fusion of 

 many rows of small teeth. Similar palatal teeth (Fig. 

 63, g) were known as fossils in the Rhaetic beds before 

 the living Ceratodus was discovered ; they occur in 

 Triassic and (very rarely) in Jurassic strata, showing 

 Ceratodus to have been marine at that time. It dis- 

 appeared from the Northern Hemisphere like Trigonia, 

 but somewhat earlier. 



Air-breathing Vertebrata. 



These are usually divided into Amphibia, Reptiles, 

 Birds, and Mammals, but palaeontological discoveries 

 have so extended the idea of a reptile that it is no longer 

 logical to separate birds from that category, so that only 

 three classes will be recognized here. 



The Amphibia are distinguished by the fact that they 

 retain signs of their aquatic ancestry by passing through 

 a larval stage in which they have gills and (with a few 



