TEE EVOLUTION OF FISHES. 561 



The few living lung fishes resemble the salamanders almost as 

 closely as they do fishes, and they may well be ranged as a class by 

 themselves midway between the primitive sharks and the amphibians. 

 The few living forms show these intermediate characters in the develop- 

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

 limbs. Those now extant give but little idea of the great variety of 

 extinct dipnoans, but the obvious suggestion that with the lung fish 

 is the place of divergence of the higher vertebrates from the fish series 

 may be the correct one. The living genera are three in number, Neo- 

 ceratodus in Australian rivers, Lepidosiren in the Amazon and Protop- 

 terus 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 environment, has thrown little light on the question of its 

 ancestry. 



From some branch of the dipnoans the ancestry of the amphibians 

 and through them that of the reptiles, birds and mammals may be 

 traced, although some reason exists for regarding the primitive Crossop- 

 terygium as the point of divergence. It may be that the Crossoptery- 

 gian gave rise to Amphibian and Dipnoan alike. 



In the process of development we next reach the characteristic 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 sup- 

 ports 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 next great offshoot is the group of crossopterygians, fishes which 



still retain the old-fashioned t}^e of pectoral and ventral fin, the 

 archipter3'gium. In the archaic tail, enameled scales and cartilaginous 

 skeleton the crossopterj'gian 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, Polypteriis, and Erpe- 

 toichthys are not very different from those which prevailed in Devo- 

 nian times. The larvae possess external gills with firm base and fringe- 

 like rays, suggesting a resemblance to the pectoral fin itself which 

 develops from the shoulder-girdle just below it and would seem to give 

 some force to Kerr's contention that the archipterygium is only a 

 modified external gill. In Polypterus the archipterygium has become 

 short and fan-shaped, its axis made of two diverging bones with flat 

 cartilage between. From this type it is thought that the arm of the 



higher forms has been developed. The bony basis may be the humerus, 

 VOL. I.X.- 36. 



