XIII 



PHYLUM CHORDATA 



229 



We see, then, that all the orders of Ganoids, during the period 

 of their prime, branched out into diverse forms, adapted to 

 different environments, and often resembling, in a remarkable 

 manner, the divergent forms of Teleostei which fill similar 

 positions at the present day. 



The Teleostei first appear in the Cretaceous rocks, where many 

 existing families are represented. From this period onwards the 

 three Ganoid orders undergo a progressive diminution in the 



FIG. 852. A, Lepidotus maximus (Jurassic), s. scale ; t. teeth. B, Caturus furcatus 



(Jurassic). (From Nicholson and Lydekker.) 



number of families, genera, and species, their places being taken 

 by the more highly differentiated Teleostei, until, at the present 

 day, as we have seen, they are reduced to a few scattered forms, 

 mostly confined to fresh waters. 



Sub-class V, The Dipnoi. 



The Dipnoi or Lung-fishes, comprising as their living repre- 

 sentatives only the Queensland Ceratodus or " Burnett Salmon," 

 and the Mud-fishes (Protopterus arid Lepidosiren) of certain South 

 African and South American rivers, are fishes of such well-marked 

 and special features that by some zoologists they are separated 

 from the true Fishes and regarded as constituting a separate class 



