[Illustration:  Fig. 39.—­Dichograptus octobrachiatus, a branched, “unicellular” Graptolite from the Skiddaw and Quebec Groups (Arenig).  (After Hall.)]

In another great group of Graptolites (including the genera Diplograptus, Dicranograptus, Climacograptus, &c.) the common stem of the colony gives origin, over part or the whole or its length, to two rows of cells, one on each side (fig. 41).  These “double-celled” Graptolites are highly characteristic of the Lower Silurian deposits; and, with an exception more apparent than real in Bohemia, they are exclusively confined to strata of Lower Silurian age, and are not known to occur in the Upper Silurian.  Lastly, there is a group of Graptolites (Phyllograptus, fig. 42) in which the colony is leaf-like in form, and is composed of four rows of cells springing in a cross-like manner from the common stem.  These forms are highly characteristic of the Arenig group.

[Illustration:  Fig. 40.—­Central portion of the colony of Didymegraptus divaricatus, Upper Llandeilo, Dumfresshire.  (Original.)]

[Illustration:  Fig. 41.—­Examples of Diplograptus pristis, showing variations in the appendages at the base.  Upper Llandeilo, Dumfriesshire. (Original.)]

[Illustration:  Fig. 42.—­Group of individuals 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.