CHAPTER XXIV.

THE EVOLUTION OF FISHES.

The Geological Distribution of Fishes.--The Earliest Sharks.--Devonian Fishes.--Carboniferous Fishes.--Mesozoic Fishes.--Tertiary Fishes.--Factors of Extinction.--Fossilization of a Fish.--The Earliest Fishes.--The Cyclostomes.--The Ostracophores.--The Arthrodires.--The Sharks.--Origin of the Shark.--The Chimæras.--The Dipnoans.--The Crossopterygians.--The Actinopteri.--The Bony Fishes. 435