THE LOWER SILURIAN PERIOD. 103 



manner from the common stem. These forms are highly char- 

 acteristic of the Arenig group. 7 



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 Ccelenterate 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 hav- 

 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, 



