hat the rocks only give the
record of shore-life, and only a part of that is as yet opened by
geology. Some experts think that they were developed in inland waters.
Reaching sometimes a length of five or six feet, with two large compound
eyes and some smaller eye-spots (ocelli), they must have been the giants
of the Silurian ocean until the great sharks and other fishes appeared.

The quaint stalked Echinoderm which also we noticed in the Cambrian
shallows has now evolved into a more handsome creature, the sea-lily.
The cup-shaped body is now composed of a large number of limy plates,
clothed with flesh; the arms are long, tapering, symmetrical, and richly
fringed; the stalk advances higher and higher, until the flower-like
animal sometimes waves its feathery arms from the top of a flexible
pedestal composed of millions of tiny chalk disks. Small forests of
these sea-lilies adorn the floor of the Silurian ocean, and their broken
and dead frames form whole beds of limestone. The primitive Cystids
dwindle and die out in the presence of such powerful competitors. Of
250 species only a dozen linger in the Silurian strata, though a new and
more advanced type--the Blastoid--holds the field for a time. It is the
age of the Crinoids or sea-lilies. The starfish, which has abandoned the
stalk, does not seem to prosper as yet, and the brittle-star appears.
Their age will come later. No sea-urchins or sea-cucumbers (which would
hardly be preserved) are found as yet. It is precisely the order of
appearance which our theory of their evolution demands.

The Brachiopods have passed into entirely new and more advanced species
in the many advances and retreats of the shores, but the Molluscs show
more interesting progress. The commanding group from the start is that
of the Molluscs which have "kept their head," the Cephalopods, and
their large shells show a most instructive evolution. The first great
representative of the tribe is a straight-shelled Cephalopod, which
becomes "the tyrant and scavenger of the Silurian ocean" (Chamberlin).
Its tapering, conical shell sometimes runs to a length of fifteen
feet, and a diameter of one foot. It would of itself be an important
evolutionary factor in the primitive seas, and might explain more than
one advance in protective armour or retreat into heavy shells. As the
period advances the shell begins to curve, and at last it forms a
close spiral coil. This would be so great an advantage that we are