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.
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