ye THE DOCTRINE OF DESCENT. 
terms them Placoids, from the rhombic scales, provided 
with a layer of enamel highly favourable to preserva- 
tion, and-covering the whole surface in oblique rows. 
FIG. 11. 
The vertebral column, as in the sharks, enters the upper 
flap of the tail and renders it strikingly unsymmetrical. 
The Ganoids are, as comparative anatomy has proved 
with certainty, a development of the shark-like fishes, if 
not decidedly of a higher grade. The Ganoids, there- 
fore, presuppose the shark. 
The carboniferous period owes its name to the enor- 
mous accumulation occurring in its midst, of the 
remains of terrestrial plants, fern-like Calamites, and 
more especially of Sigillaria and Lepidodendra, stand- 
ing between vascular Cryptogams and Conifers. They 
formed tropical bog-forests, such as Franz Unger some 
years ago attempted to restore in an ingenious compo- 
sition. In these steaming primeval forests, differing 
from the early beginnings of antecedent periods by 
their extent and luxuriance, new phases of animal 
life become manifest—scorpions, myriapods, and in- 
sects—in other words, air-breathing Articulata, and 
likewise the first air-breathing Vertebrata. The latter, 
