THE PRIMARY EPOCH. II 
the forest vegetation of the mainland, which was then 
utterly wanting. All the animals, also, whose remains have 
been found in archilithic strata, like the plants, lived in 
water. Only crustacea are met with among the animals 
with articulated feet, as yet no spiders and no insects. Of 
vertebrate animals, only a very few remains of fishes are 
known as having been found in the most recent of all 
primordial strata, in the upper Silurian. But the headless 
vertebrate animals, which we call skull-less, or Acrania, and 
out of which fishes must have been developed, we suppose 
to have lived in great numbers during the primordial epoch. 
Hence we may call it after the Acrunia as well as after the 
Tangles. 
The primary epoch, or the era of Fern Forests, the second 
main division of the organic history of the earth, which is 
also called the palzolithic or paleeozoic period, lasted from 
the end of the Silurian formation of strata to the end of the 
Permian formation. This epoch was also of very long dura- 
tion, and again falls into three shorter periods, during which 
three great systems of strata were deposited, namely, first, 
the Devonian system, or the old red sandstone; upon that, 
the Carboniferous, or coal system; and upon this, the 
Permian system. The average thickness of these three 
systems taken tovether may amount to about 42,000 feet, 
from which we may infer the immense length of time 
requisite for their formation. 
The Devonian and Permian formations are especially rich 
in remains of fishes, of primeval fish as well as enamelled 
fish (Ganoids), but the bony fish (Teleostei) are absent from 
_ the strata of the primary epoch. In coal are found the 
most ancient remains of animals living on land, both of arti- 
