THE LOWER SILURIAN PERIOD. 



103 



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 



Fig. 40. Central portion of the colony of Didymograptus divaricatus, Upper 

 Llandeilo, Dumfriesshire. (Original.) 



length, to two rows of cells, one on each side (fig. 41). These 

 " double-celled " Graptolites are highly characteristic of the 



Fig. 41. Examples of Diplograptus Fig. 42. Group of individuals of Phyllo- 



prtetis, showing variations in the append- graptus typus, from the Quebec group of 



ages at the base. Upper Llandeilo, Canada. (After Hall.) One of the four rows 



Dnmfreisshire. (Original.) of cells is hidden on the under surface. 



Lower Silurian deposits; and, with an exception more appar- 



