are found fossil in great numbers; the teeth are in the form of plates, giving the appearance of a pavement. The only Shark at the present day having such teeth is the Cestracion, or Port Jackson Shark, confined to the Australian and China seas. The Ganoids, so called from their shining plates or scales, must have abounded in the Devonian seas, from the numerous fossil genera and species that have been described. The only living examples of Ganoids at the present time are the Sturgeons, Gar-pike, Amia of North America, and the Polypterus of the Nile. In the chapter on Zoology we argued, from their structure, that the Sharks and Ganoids were not so highly organized as the Teliosts, or bony fish of the present day, and concluded that therefore the Sharks and Ganoids had preceded the Teliosts in their appearance on the earth. This view is confirmed by what we have just seen, that the fishes that first appeared were Sharks and Ganoids. Further, we noticed that the Ganoids, while intermediate in many respects between the Sharks and Teliosts, have many striking affinities with the Batrachia and Reptilia. The fact of the Ganoids appearing before the Bony Fish and Batrachia is a striking confirmation of the truth of the view proposed, that the Ganoids were the common stock from which the stems of the Teliosts and Batrachia diverged. Calling attention to the fact of the Silurian period, or Age of Mollusca, preceding the Devonian period, or Age of Fishes, being in harmony with the view of the higher forms of life coming from the lower, we pass on to the Carboniferous period.
AGE OF ACROGENS AND BATRACHIA.
Pennsylvania, the great coal State, was principally formed during the Carboniferous period, often called the Age of Acrogens or Summit-growers, from eight-tenths of its plants belonging to that order of the vegetal kingdom.
