ern States. It is possible to
read from the lowest rock formations upward, the rise of these sea
giants and their gradual decline. Certain strata of limestone contain
the last relics of this race, after which they became extinct. As the
straight-chambered forms diminished, great coiled forms became more
abundant, but all died out.

One of the most abundant fossil animals in ancient rocks is called a
trilobite. Its body is divided by two grooves into three parts, a
central ridge extending the whole length of the body and two side
ridges. The front portion of the shell formed the head shield, and
behind the main body part was a little tail shield. The skeleton was
formed of many movable jointed plates, and the creature had eyes set in
the head shield just as the king crab's are set. Jointed legs in pairs
fringed each side of the body. Each leg had two branches, one for
walking, the other for swimming. A pair of feelers rose from the head.
The body could be rolled into a ball when danger threatened, by bringing
head and tail together.

These remarkable, extinct trilobites were the first crustaceans. Their
nearest living relative to-day is the horseshoe crab. The fresh-water
crayfish and the lobster are more distant relatives: so are the shrimps
and the prawns. No such abundance of these creatures exists to-day as
existed when the trilobites thronged the shallows. So well preserved are
these skeletons that, although there are no living trilobites for
comparison, it is possible to find out from the fossil enough about
their structure to know how they fed and lived their lives along with
the straight-horns which were the scavengers of those early seas and the
terror of smaller creatures. The trilobites throve, and, dying, left
their record in the rocks; then disappeared entirely. We find their
fossils in a great variety of forms, shapes, and sizes. The smallest is
but a fraction of an inch long, the largest twenty inches long.

The ancient rocks, in which these lower forms of life have left their
fossils, are known as the Silurian system. The time in which these rocks
were accumulating under the seas covers a vast period. We call it the
Age of Invertebrates, because these soft-bodied, hard-shelled animals,
the crinoids, the molluscs, and the trilobites, with bony external
skeletons and no backbones, were the most abundant. They overshadowed
all other forms of life. The rocks of this wonderful series were formed
on the shor