xm PHYLUM CHORDATA 239 



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 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 IV. The Dipnoi. 



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

 sentatives only the Queensland Cemtodus or " Burnett Salmon," 

 and the Mud-fishes (Protoptems and 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 

 of Vertebrates. One of their peculiar features is indicated by the 

 name Dipnoi. Not only do these animals breathe by means of 

 gills, like ordinary Fishes, but they have a highly-developed 

 apparatus for the respiration of air a lung or lungs with an 

 arrangement of the circulation co-ordinated with this, such as is 

 indicated in Polypterus and Amia only among the Teleostomi. 

 They have bony scales and dermal fin-rays, but the paired fins, 

 unlike those of any other Fishes, with the exception of certain 

 extinct Elasmobranchs, are constructed on the biserial type 

 (" archipterygium," see p. 163). 



1. EXAMPLE OF THE CLASS Cemtodus (Neocemtodus 

 or Epiceratodus) forsteri. 



The Ceratodus or " Burnett Salmon " (Fig. 914) is the largest 

 of the Dipnoi, attaining a length sometimes of four or five feet. 

 It occurs at the present day only in the Burnett and Mary Rivers 

 in Queensland, but fossil teeth referred to the same or nearly related 

 genera have been found in abundance in Palaeozoic and Mesozoic 

 beds in Europe, America, the East Indies, Africa, and Australia. 

 Ceratodus forsteri lives in still pools in which the water in the dry 

 season becomes extremely stagnant and overladen with decom- 

 posing vegetable matter ; and at that season it is only by rising to 

 the surface occasionally, and taking air into its lung, that it is 

 enabled to obtain sufficient oxygen for purposes of respiration. 

 Its food consists of such small animals as live among the water- 

 plants and decaying leaves, and in order to obtain a sufficient amount 

 of such food, it swallows relatively large quantities of vegetable 



