450 The Evolution of Fishes 



characteristic appendage. The shark-like structure of the 

 mouth remains. 



The few living lung-fishes resemble the salamanders in 

 many regards, and some writers have ranged the class as 

 midway between the primitive sharks and the amphibians. 

 These forms show their intermediate characters in the develop- 

 ment of lungs and in the primitive character of the pectoral and 

 ventral limbs. Those now extant give but little idea of the 

 great variety of extinct Dipnoans. The living genera are three 

 in number Neoceratodus in Australian rivers, Lepidosiren in 

 the Amazon, and Protopterus in the Nile. These are all mud- 

 fishes, some of them living through most of the dry season 

 encased in a cocoon of dried mud. Of these forms Neoceratodus 

 is certainly the nearest to the ancient forms, but its embryology, 

 owing to the shortening of its growth stages due to its environ- 

 ment, has thrown little light on the question of its ancestry. 



From some ally of the Dipnoans the ancestry of the am- 

 phibians and through them that of the reptiles, birds, and mam- 

 mals may be traced, although a good deal of evidence has 

 been produced in favor of regarding the primitive crossop- 

 terygian or fringe fin as the point of divergence. It is not un- 

 likely that the Crossopterygian gave rise to Amphibian and 

 Dipnoan alike. 



In the process of development we next reach the charac- 

 teristic fish mouth in which the upper jaw is formed of maxillary 

 and premaxillary elements distinct from the skull. The upper 

 jaw of the shark is part of the palate, the palate being fused 

 with the quadrate bone which supports the lower jaw. That 

 of the Dipnoan is much the same. The development of a typical 

 fish mouth is the next step in evolution, and with its appearance 

 we note the decline of the air-bladder in size and function. 



The Crossopterygians. The fish-like mouth appears with the 

 group of Crossopterygians, fishes which still retain the old- 

 fashioned type of pectoral and ventral fin, the archipterygium. 

 In the archaic tail, enameled scales, and cartilaginous skeleton 

 the Crossopterygian shows its affinity with its Dipnoan ancestry. 

 Thus these fishes unite in themselves traits of the shark, lung- 

 fish, and Ganoid. The few living Crossopterygians, Polypterus 

 and Erpetoichthys, are not very different from those which pre- 



