100 HISTORICAL PALZONTOLOGY. 
Silurian deposits by a great number of forms belonging on the 
one hand to the true Corals, and on 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 (2 g., Ductyonema, Dendrograptus, &c.), the 
family of the Grapéolites 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 ap- 
pearance, 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 Grap- 
tolites (Gr. grapho, I write; ¢thos, stone) were so named by 
Linneeus, from the resemblance of some of them to written or 
pencilled marks upon the stone, though the great naturalist him- 
self did not believe them to be true fossils at all. They occur 
as linear or leaf-like bodies, sometimes simple, sometimes com- 
pound and branched; and no doubt whatever can be enter- 
tained 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 deli- 
cate 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 Grafio- 
lite, 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 ongin 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 AZonograptus) ; 
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 
