Jul 
GEOLOGY. 115 
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 Ces- 
tracion, 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 there- 
fore 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 confirma- 
tion 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. 
