lt by these
lime-building polyps. Their forms are so well preserved in the rocks
that it is possible to know just how they looked when they grew in the
shallows.

One very common kind is called a cup coral, because the polyp formed a
skeleton shaped like a cup. The body wall surrounded the skeleton, and
the arms or tentacles rose from the centre of the funnel-like depression
in the top. Little cups budded off from their parents, but remained
attached, and at length the skeletons of all formed great masses of limy
rock. Some cup corals grew in a solid mass, the new generation forming
an outer layer, thus burying the parent cups.

A second type of corals in these oldest limestones is the honeycomb
group. The colonies of polyps lived in tubes which lengthened gradually,
forming compact, limy cylinders like organ pipes, fitted close together.
The living generation always inhabited the upper chambers of the tubes.
A third type is the chain coral, made of tubes joined in rows, single
file like pickets of a fence. But these walls bend into curious
patterns, so that the cross-section of a mass of them looks like a
complex pattern of crochet-work, the irregular spaces fenced with chain
stitches. Each open link is a pit in which a polyp lived.

Among the corals are sprays of a fine feathery growth embedded in the
limestone. Fine, straight, splinter-like branches are saw-toothed on one
or both edges. These limy fossils might not be seen at all, were they
not bedded in shales, which are very fine-grained. Here again are the
skeletons of animals. Each notch on each thread-like branch was the home
of a tiny animal, not unlike a sea anemone and a coral polyp.

To believe this story it is necessary only to pick up a bit of dead
shell or floating driftwood on which a feathery growth is found. These
plumes, like "sea mosses," as they are called, are not plants at all,
but colonies of polyps. Each one lived in a tiny pit, and these pits
range one above the other, so as to look like notches on the thread-like
divisions of the stem. Put a piece of this so-called "sea moss" in a
glass of sea water, and in a few moments of quiet you will see, by the
use of a magnifying glass, the spreading arms of the polyp thrust out of
each pit.

The ancient seas swarmed with these living hydrozoans, and their remains
are found preserved as fossils in the shales which once were beds of
soft mud.

The hard shells of sea urchins and starfishes are made