THE LOWER. SILURIAN PERIOD. IOL 
common point, as in the so-called “twin Graptolites ” (Dzdy- 
mograptus, fig. 40). This type is entirely confined to the earlier 
portion of the Lower Silu- 
rian period (Arenig and 
Llandeilo). In other cases, 
again, there may be four 
of such stems springing 
from acentral point (Zé?- 
ragraplus). Lastly, there 
are numerous complex 
forms (such as Dichograp- 
tus, Loganograptus, &c.) 
which there are eight or 
more of these simple bran- 
ches, all arising from a 
common centre (fig. 39), 
which is sometimes fur- 
nished with a_ singular 
horny disc. These com- 
plicated branching forms, 
as well as the Zetragrapiz, 
are characteristic of the 
horizon of the Arenig 
group. Similar forms, of- 
ten specifically identical, 
Fig. 39.—Dichograptus octobrachiatus, a branched, “unicellular” Graptolite from 
the Skiddaw and Quebec Groups (Arenig). (After Hall.) 
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. 
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 of its 
length, to ‘wo 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 appa- 
