DIPNOI. 



273 



DiPNEUMONiA. In the first, typified by the Austrahan- genus 

 Ceratodus, there is but a single air-bladder (lung), and the fins 

 have the secondary rays of the archipterygium (Fig. 185) well 

 developed. Ceratodus forsteri of Australia attains a length of 

 five feet ; it lives in fresh water in places where it is apt to be- 

 come stagnant, and at such times calls its lung into function. 



Fig. 275. Lung-fish, Protopterus aimectans^ from Boas. 



The genus Ceratodus also occurs fossil in the triassic and Juras- 

 sic of Europe, India, and Colorado, the peculiar dental plates 

 being very characteristic (Fig. 271). The Dipneuraonia have 

 two air-bladders, and the paired fins retain only the axial part of 

 the archipterygial skeleton (Fig. 269). The living genera are 

 Protopterus from African rivers, and Lepidosirejt from South 

 America. At the time of drought the African form burrows 

 into the mud at the bottom of the pools 

 where it lives, and by the aid of the mucus 

 from its body forms the earth into a ' co- 

 coon,' in which it lives in a state of sus- 

 pended animation until the return of the 

 rainy season. 



Allied to these living animals are a num- 

 ber of fossil forms characterized by the pres- 

 ence of numerous plates in the cranial wall. 

 These occur in the palaeozoic rocks. In 

 Dipterus and Phaneropleuron from the De- 

 vonian of Europe and America, jugular 

 plates are present ; in Ctenodus and Sageno- 

 dus, from the carboniferous of both hemispheres, jugulars are 

 lacking. 



Fig. 276. Dorsal 

 view of skull of Dip- 

 terus, after Pander. 



