EVOLUTION AND PALAEONTOLOGY 257 



gravity of the fish. In the evolution of these forms we can 

 trace a gradual return to the sea, for among existing fishes the 

 more primitive forms live more or less completely in fresh 

 water, and the most recent and most modified are typically 

 marine. The fins of Actinopterygians are characterised by the 

 reduction of the basal lobe and its internal skeleton, the dermal 

 rays arising from the surface of the body. The oldest form is 

 Chirolepis which occurs in the Lower Old Red Sandstone 

 together with the earliest Crossopterygians. This genus belongs 

 to the group or branch of the Chondrostei and is the first of 

 the great family Palaeoniscidas which developed an abundance 

 of species and individuals in the Carboniferous and Permian 

 formations and gradually diminished in the Secondary period 

 till the end of the Jurassic. These fishes had a complete armour 

 of ganoid scales, a large heterocercal tail, and a single dorsal fin 

 in the middle of the back. The dermal bones of the head were 

 external and were simply the ganoid plates of that region. 

 As the Palasoniscidaf are disappearing the family Chondros- 

 teidas comes into existence, distinguished by the degeneration 

 of the scales on the body ; specimens occur in the Lias or 

 Lower Jurassic, and these forms must be regarded as directly 

 descended from the Palseoniscidas. The Chondrosteidae, on the 

 other hand, are the ancestors of the existing Sturgeons, which 

 are not known before the Tertiary. A branch from the Palas- 

 oniscidae gave rise to the Platysomidaa which existed in the 

 Carboniferous and Permian periods ; they are deep-bodied 

 compressed fishes completely covered with ganoid scales, but 

 with short jaws carrying blunt crushing teeth ; they do not 

 appear to have given rise to any later forms. 



The Holostei seem to have been derived from the Palaeonis- 

 cidae through the Catopteridae which are small fishes found in 

 the Triassic rocks of North America, Europe, and Australia ; 

 they have the ganoid scaling, but the dermal fin-rays of the 

 median fins are almost reduced to the number of the supporting 

 radials as in the Holostei, and the upper lobe of the tail is 

 diminished, so that the homocercal condition is approached. 

 The Holostei are a large and diversified group which include 

 forms that are transitional to the more primitive Teleostei. 

 The more primitive and earlier forms retain the ganoid scaling, 

 while some of the later have cycloid scales and perfectly homo- 

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