DIPNOI. 



273 



In the first, typified by the Australian genus 

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

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

 developed. Ccratodus 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. 



FlG. 275. Lung-fish, Protopterus annectans, from Boas. 



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

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

 being very characteristic (Fig. 271). The Dipneumonia 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 Lepidosiren 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- > FlG - 2 76. Dorsal 

 vonian of Europe and America, jugular s ^ftcr Pander 

 plates are present ; in Ctenodus and Sageno- 

 dns, from the carboniferous of both' hemispheres, jugulars are 

 lacking. 



