of _Phyllograptus
typus_, from the Quebec group of Canada. (After Hall.) One of
the four rows of cells is hidden on the under surface.]

The Graptolites are usually found in dark-coloured, often black
shales, which sometimes contain so much carbon as to become
"anthracitic." They may be simply carbonaceous; but they are
more commonly converted into iron-pyrites, when they glitter
with the brilliant lustre of silver as they lie scattered on the
surface of the rock, fully deserving in their metallic tracery
the name of "written stones." They constitute one of the most
important groups of Silurian fossils, and are of the greatest
value in determining the precise stratigraphical position of
the beds in which they occur. They present, however, special
difficulties in their study; and it is still a moot point as
to their precise position in the zoological scale. The balance
of evidence is in favour of regarding them as an ancient and
peculiar group of the Sea-firs (Hydroid Zoophytes), but some
regard them as belonging rather to the Sea-mosses (_Polyzoa_).
Under any circumstances, they cannot be directly compared either
with the ordinary Sea-firs or the ordinary Sea-mosses; for these
two groups consist of fixed organisms, whereas the Graptolites
were certainly free-floating creatures, living at large in the
open sea. The only Hydroid Zoophytes or Polyzoans which have
a similar free mode of existence, have either no skeleton at
all, or have hard structures quite unlike the horny sheaths of
the Graptolites.

The second great group of Coelenterate animals (_Actinozoa_)
is represented in the Lower Silurian rocks by numerous Corals.
These, for obvious reasons, are much more abundant in regions
where the Lower Silurian series is largely calcareous (as in
North America) than in districts like Wales, where limestones
are very feebly developed. The Lower Silurian Corals, though
the first of their class, and presenting certain peculiarities,
may be regarded as essentially similar in nature to existing
Corals. These, as is well known, are the calcareous skeletons of
animals--the so-called "Coral-Zoophytes"--closely allied to the
common Sea-anemones in structure and habit. A _simple_ coral (fig.
43) consists of a calcareous cup embedded in the soft tissues of
the flower-like polype, and having at its summit a more or less
deep depression (the "calice") in which the digestive organs
are contained. The space within the coral is divided