The above gives the elementary constitution of any _Graptolite_, but there are considerable differences as to the manner in which these elements are arranged and combined. In some forms the common stem of the colony gives origin to but a single row of cells on one side. If the common stem is a simple, straight, or slightly-curved linear body, then we have the simplest form of Graptolite known (the genus _Monograptus_); and it is worthy of note that these simple types do not come 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.)]