428 EVOLUTIONARY MORPHOLOGY 



Devonian and gave rise to the Holocephali, represented at the 

 present day by Chimaera. 



The bony fish or Osteichthyes appear in the Devonian, 

 and the lungs which they possess were probably in connexion 

 with the poor oxygen- content of the fresh water in which they 

 lived. The Dipnoi were represented by Dipterus (Devonian), 

 and the non-Dipnoan bony fish, or Teleostomi were represented 

 by Osteolepis (also Devonian). These two forms were closely 

 related, and they had the following characters in common : 

 blunt lobate fins, a pair of external and a pair of internal 

 nostrils, the general arrangement of the bones of the roof of the 

 skull, heterocercal tails, and, most important of all, cosmoid 

 scales. There is no doubt that they had a common ancestor, 

 perhaps in the Silurian, and from a close relative of this 

 ancestor the Tetrapods arose. One of the Osteolepidoti, 

 vSauripterus, had fins from which the structure of the penta- 

 dactyl limb of the Tetrapod might be derived. In the Ccela- 

 canths, which are Teleostomes related to the Osteolepidoti, 

 there is definite evidence of the presence of a lung, for it was 

 calcified and fossilised. The Dipnoi evolved into the forms 

 living at the present day, and became more and more adapted 

 to life in rivers which are liable to dry up. The wide and dis- 

 continuous distribution of Ceratodus (Australia), Lepidosiren 

 (South America) and Protopterus (Africa) to-day is evidence 

 of the antiquity of the group. The evolution of these forms 

 went on parallel to that of the early Tetrapods, and inde- 

 pendently from them. 



On the Teleostome side, another group arose in the 

 Devonian from some relatives of the Osteolepidoti : the 

 Palaeoniscoidea. These fish are characterised by the possession 

 of scales of the type called palseoniscoid . Cheirolepis resembled 

 Osteolepis in the structure of its skull, but its eyes were larger 

 and the heterocercal tail was more accentuated. This pro- 

 vision for more active swimming was probably connected with 

 the improvement of the eyes as sense-organs. Polyp terus, 

 alive to-day, may be regarded as a descendant of the Palaeo- 

 niscoids. It has palaeoniscoid scales, and preserves the open 

 spiracle. It inhabits certain rivers in Africa. On the other 



