The Evolution of Fishes 



449 



covered Rhinochimcera of Japan. The fusion of the teeth into 

 overlapping plates, the covering of the gills by a dermal flap, 

 the complete union of the palatoquadrate apparatus or upper 

 jaw with the skull and the development of a peculiar clasping 



FIG. 258. A Deep-sea Chimsera, Harriotta raleighiana Goode & Bean. 



Gulf Stream. 



spine on the forehead of the male are characteristic of the Chi- 

 maeras. The group is one of the most ancient, but it ends 

 with itself, none of the modern fishes being derived from 

 Chimaeras. 



The Dipnoans. The most important offshoot of the primitive 

 sharks is not the Chimaeras, nor even the shark series itself, but 

 the groups of Crossopterygians and Dipnoans, or lung-fishes, with 

 the long chain of their descendants. With the Dipnoan appears 



FIG. 259. An extinct Dipnoan, Dipierus valenciennesi Agassiz. 



(After Pander.) 



Devonian. 



the lung or air-bladder, at first an outgrowth from the ventral 

 side of the oesophagus, as it still is in all higher animals, but 

 later turning over, among fishes, and springing from the dorsal 

 side. At first an arrangement for breathing air, a sort of 

 accessory gill, it becomes the sole organs of respiration in 

 the higher forms, while in the bony fishes its respiratory function 

 is lost altogether. The air-bladder is a degenerate lung. In the 

 Dipnoans the shoulder-girdle moves forward to the skull, and 

 the pectoral limb, a jointed and fringed archipterygium, is its 



