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 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.