o-day, not a single member
of the frog and salamander class, not a reptile, not a bird, not a
mammal, and probably no air-living insects. It is highly doubtful
whether there was any animal living upon the land and breathing the
air twenty-five million years ago.

We start our study, then, at the period known as the Palaeozoic era,
the era of the ancient life of the globe, beginning twenty-five
million and ending ten million years ago. The first of the three
sections into which this period of life is divided is known as the
Silurian age, the age of invertebrates. The word invertebrate is an
unscientific but convenient term under which we embrace all the
animals below those having backbones. This period is called the age of
invertebrates because, although there is an enormous wealth of animal
and plant life in the Silurian, there are no backboned animals except
the lowest kinds of fishes. It was supposed for a long time that even
fishes were absent. Now we know they existed, but they were small and
inconspicuous. In this period corals were wonderfully abundant,
particularly in the great internal sea which spread over what is now
known as the Mississippi Valley. Everywhere over this region must have
grown in the shallow water great numbers of creatures called crinoids
or stone lilies. They were attached to the bottom by slender stems,
sometimes many feet long. These stems are jointed, and when they
became fossilized the sections were apt to separate, with the result
that over a wide area in the Mississippi Valley it is very common to
find these little segments which look not unlike checkers. At the end
of the stem was a rounded head, with a mouth at the top, and around
the mouth were branched, feathery arms. The creatures must have been
exquisitely beautiful, but they have completely disappeared from the
face of the earth, with the exception of a very few, found in the
obscurity of the almost fathomless depths of the great ocean. Here
they remain as peculiar relics, only preserved by the unvarying
conditions in the deep sea from the extinction that has met their
sisters.

Those who are familiar with our seacoast will know an interesting
creature known as the horseshoe crab, or king crab, though in reality
it is not a crab at all. It is rather more nearly related to the
spiders than the crabs, though no one but a technical zooelogist could
possibly associate them together. The ancestors of these king crabs
were the fines