f forms belonging on the one hand to the true
Corals, and en the other hand to the singular family of the
_Graptolites_. If we except certain plant-like fossils which
probably belong rather to the Sertularians or the Polyzoans (e.g.,
_Dictyonema, Dendrograptus_, &c.), the family of the _Graptolites_
may be regarded as exclusively Silurian in its distribution. Not
only is this the case, but it attained its maximum development
almost upon its first appearance, in the Arenig Rocks; and whilst
represented by a great variety of types in the Lower Silurian;
it only exists in the Upper Silurian in a much diminished form.
The _Graptolites_ (Gr. _grapho_, I write; _lithos_, stone) were
so named by Linnaeus, from the resemblance of some of them to
written or pencilled marks upon the stone, though the great
naturalist himself did not believe them to be true fossils at
all. They occur as linear or leaf-like bodies, sometimes simple,
sometimes compound and branched; and no doubt whatever can be
entertained as to their being the skeletons of composite organisms,
or colonies of semi-independent animals united together by a common
fleshy trunk, similar to what is observed in the colonies of the
existing Sea-firs (Sertularians). This fleshy trunk or common
stem of the colony was protected by a delicate horny sheath, and
it gave origin to the little flower-like "polypites," which
constituted the active element of the whole assemblage. These
semi-independent beings were, in turn, protected each by a little
horny cup or cell, directly connected with the common sheath
below, and terminating above in an opening through which the
polypite could protrude its tentacled head or could again withdraw
itself for safety. The entire skeleton, again, was usually, if
not universally, supported by a delicate horny rod or "axis,"
which appears to have been hollow, and which often protrudes to
a greater or less extent beyond one or both of the extremities
of the actual colony.

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