THE EVOLUTION THEORY 



107 



brates met with in the Silurian Period, a low species of fishes 

 related to the sharks. This class assumes greater importance in 

 the Devonian Period. They are mostly fantastic forms, widely 

 differing from present species, called Placodermata on account 

 of the powerful osseous armour which covers the anterior 

 half of their body. In the Devonian formation are also found 

 transition-forms from the fishes to the amphibians, the so-called 

 lung-fishes or Dipnoi. Species of this remarkable Order still 

 exist, dispersed over the whole world. Thus we find on the 



FIG. 26. SHELLS OF EXTINCT CEPHALAPODA. 



(1) Orthoceros timidum (Upper Silurian) ; (2 and 3) Ceratites 

 nodosus (Upper Muschelkalk, Triassic), front and side view. 



river courses of the Australian continent the well-known Ceratodus 

 forsteri, in Africa the Protopterus annectens, and in South America 

 a third species, Lepidosiren paradoxa. 



The most important distinctive feature of the Dipnoi is 

 that, unlike fishes, they are not restricted to breathing through 

 gills. For if the waters which they inhabit should dry up the 

 gills cease to be active and the swim-bladder becomes a respira- 

 tory organ, a lung. As we must probably trace the amphibians 

 phylogenetically from similar forms, it is important to find 

 that they are preceded by them in the geological formations. 

 Whether the fossil-ancestors of the Dipnoi already possessed 

 lung-respiration must be regarded as doubtful. Their distribu- 



