into existence till
comparatively late (Llandeilo), and last nearly to the very close
of the Upper Silurian. In other cases, whilst there is still but
a single row of cells, the colony may consist of two of these
simple stems springing from a common point, as in the so-called
"twin Graptolites" (_Didymograptus_, fig. 40). This type is entirely
confined to the earlier portion of the Lower Silurian period
(Arenig and Llandeilo). In other cases, again, there may be four
of such stems springing from a central point (_Tetragraptus_).
Lastly, there are numerous complex forms (such as _Dichograptus,
Loganograptus_, &c.) in which there are eight or more of these
simple branches, all arising from a common centre (fig. 39),
which is sometimes furnished with a singular horny disc. These
complicated branching forms, as well as the _Tetragrapti_, are
characteristic of the horizon of the Arenig group. Similar forms,
often specifically identical, are found at this horizon in Wales,
in the great series of the Skiddaw Slates of the north of England,
in the Quebec group in Canada, in equivalent beds in Sweden, and
in certain gold-bearing slates of the same age in Victoria in
Australia.

[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