522 PISCES— FISHES. 



It is likely that some of the loosely-built deep-sea fishes, such as the 

 pelican fish, Eurypharynx, are not referable to the orders usually recog- 

 nised. 



Dipnoi. " Mud-Fishes." 



The Dipnoi, whose name means double breathers, are 

 now represented by three genera — Ceratodus, from two 

 rivers of Queensland ; Protopterus, from certain African 

 rivers, e.g. the Gambia ; and Lepidosiren, from the Amazons. 

 The wide distribution is noteworthy. 



They are very ancient forms, for Ceratodus, or a closely 

 allied form, has lived on from Mesozoic times, and there 

 were also undoubted Dipnoi far back in Palaeozoic times, 

 such as Dipterus and Phaneropleuron of the Devonian, 

 Ctenodus and Uronemus of the Carboniferous. According 

 to some, the remarkable Devonian Coccosteidae are also to 

 be considered as an aberrant group of Dipnoi. 



Prof. W. N. Parker regards them as " the isolated sur- 

 vivors of an exceedingly ancient group, which was probably 

 nearly allied to the ancestors of existing Amphibians and 

 Fishes, more particularly Elasmobranchs, though the Ganoid 

 stock most likely arose not far off." 



Were it not for the disadvantage of multiplying classes, 

 one would be inclined to place them between Pisces, which 

 they resemble in having cycloid scales, paired fins, a spiral 

 valve, etc., and Amphibia, which they approach in having 

 lungs, an incipiently three-chambered heart, a vena cava, 

 a pulmonary vein, posterior nares, and multicellular skin 

 glands. 



The Dipnoi are physiologically transitional between Fishes 

 and Amphibians, having, for instance, acquired lungs while 

 retaining gills, but it does not follow that they are morpho- 

 logically transitional. They are intermediate, but that is not 

 to say that they are the connecting links. To mention only 

 one point, there is little or no evidence that the Dipnoan 

 swim-bladder or lung is homologous with the Amphibian 

 lung. 



Ceratodus. — The genus Ceratodus is abundantly repre- 

 sented by fossils in the Mesozoic beds of Europe, America, 

 Asia, and Australia, but the living animal is now limited to 

 the basins of the Burnett and Mary rivers of Queensland. 



