The Evolution of Fishes 



437 



of the Carboniferous. The Arthrodires also increase greatly 

 in variety and in size, reaching their culmination in the De- 

 vonian, but not disappearing entirely until well in the Carbon- 

 iferous. These two groups (often united by geologists under 

 the older name Placoderms) together with sharks and a few 

 Chimaeras made up almost exclusively the rich fish-fauna of 

 Devonian times. The sharks were chiefly Acanthodean and 

 Psammodont, as far as our records show. The supposed more 

 primitive type of Cladoselache is not known to appear before 

 the latter part of the Devonian Age, while Pleiiracanihus and 

 Cladodns, sometimes regarded as still more primitive, are as yet 

 found only in the Carboniferous. It is clear that the records 

 of early shark life are still incomplete, whatever view we may 

 adopt as to the relative rank of the different forms. Chimaeroids 

 occur in the Devonian, and with them a considerable variety 

 of Crossopterygians and Dipnoans. The true fishes appear also 

 in the Devonian in the guise of the Ganoid ancestors and rela- 

 tives of Palcconisciim, all with diamond-shaped enameled scales. 

 In the Devonian, too, we find the minute creature Palceospon- 

 dyliis, our ignorance of which is concealed under the name 

 Cyclia:. 



Carboniferous Fishes. — In the Carboniferous Age the sharks 

 increase in number and variety, the Ostracophores disappear, and 



Fig. 248. — Dipterus valenciennesi Af!,aiifi\z, a, Dipnoan 



Woodward.) 



(After Dean, from 



the Arthrodires follow them soon after, the last being recorded 

 from the Permian. Other forms of Dipnoans, Crossopterygians, 

 and some Ganoids now appear giving the fauna a somewhat more 

 modem aspect. The Acanthodei and the Ichthyotonn pass away 

 with the Permian, the latest period of the Carboniferous Age. 



Mesozoic Fishes. — In the Triassic period which follows the 

 Permian, the earliest types of Ganoids give place to forms ap- 



