THE LOWER SILURIAN PERIOD. 



lOI 



common point, as in the so-called '' twin Graptolites " (Didy- 

 mograptiis, 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 a central point ( Tet- 

 ragrapius). Lastly, there 

 are numerous complex 

 forms (such as Dichograp- 

 tus, Loganograpttis, &c.) in 

 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 Tetragrapti, 

 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 

 Diplograptiis^ Dicranograptus, Cltjuacograptus, ^c.) the common 

 stem of the colony gives origin, over part or the whole of its 

 length, to two 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- 



