102 HISTORICAL PALEONTOLOGY. 



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 " (Didy- 

 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 a central point. (Tet- 

 ragraptus) . Lastly, there 

 are numerous complex 

 forms (such as Dichograp- 

 tus, Loganograptus, &c.) in 

 which there are eight or 

 more of these simple bran- 

 ches, all arising from a 

 common center (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 octobracMatus,'& branched, "unicellular" Grapto/lte from 

 the Skidd aw 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. 



