THE LOWER SILURIAN PERIOD. 103 
manner from the common stem. These forms are highly char- 
acteristic of the Arenig group. 
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 posi- 
tion 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 Poly- 
zoans 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 ike Wales, where 
limestones are very feebly developed. The Lower Silunan 
Corals, though the first of their class, and presenting certain 
pecuharities, 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 strugture and 
habit. A szmple coral (fig. 43) consists of a calcareous cup 
embedded in the soft tissues of the flower-like polype, and hay- 
ing 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 into compartments by numerous vertical 
calcareous plates (the “‘ septa”), which spring from the inside 
of the wall of the cup, and of which some generally reach the 
centre. Compound corals, again (fig. 44), consist of a greater 
or less number of structures similar in structure to the above, 
