256 FISHES 



while the Crossopterygian was predatory and developed the teeth 

 on the external dermal jaw-bones. A further step in the direction 

 of air-breathing habits led to the Amphibia, but we cannot de- 

 rive these directly from the Dipnoan because they have well- 

 developed tooth-bearing maxillae and premaxillae, while on the 

 other hand they differ from the Crossopterygian in having the 

 upper jaw fixed directly to the skull. It is best to conclude that 

 the three types, Crossopterygian, Dipnoan, and Amphibian, 

 arose by divergence from the original Elasmobranch fishes 

 which came from the sea to the fresh waters of the Palaeozoic 

 period. The existing Dipnoans have degenerated in the struc- 

 ture of limbs and tail ; the heterocercal tail is adapted to 

 vigorous motion and to the habit of feeding from the ground or 

 of plunging into the depths, for the lower lobe of the tail being 

 most developed throws the tail upwards and the head down- 

 wards. The Dipnoi living in narrow waters are sluggish in 

 habit and consequently the tail fin has degenerated and a 

 secondary symmetrical tail has been formed from dorsal and 

 ventral fin-rays, the paired fins are mostly slender and rudi- 

 mentary, and the body has become elongated and eel-like. In 

 the Secondary or Mesozoic period the Dipnoi appear to have 

 been represented by Ceratodus or a form very slightly different, 

 and the only remains of these fish which have been preserved 

 are the dental plates or the parts of the skull ; these have been 

 found in almost all parts of the world. 



We cannot trace a complete series of intermediate stages 

 from the palaeozoic Crossopterygians to the existing forms of 

 the same type, namely the African Polypterus and Calamo- 

 ichthys. Polypterus has rhombic ganoid scales like those of 

 the Devonian Osteolepis ; unfortunately no fossil Crossoptery- 

 gians are known from Tertiary formation, and those of the 

 Secondary period called Ccelacanthidae are less primitive in 

 their scales and other structures than Polypterus ; the scales of 

 Ccelacanthidae are cycloid, the terminal part of the tail is dis- 

 appearing and the basals of the two dorsal and single ventral fins 

 are united into a single, forked bone. From the Crossoptery- 

 gians are derived the ray-finned fishes (Actinopterygians), in 

 which the lungs gradually lose their respiratory function and 

 paired character, and become more and more completely 

 adapted to serve as an air-bladder which diminishes the specific 



