THE LOWER SILURIAN PERIOD. 



103 



In another great group of Graptolites (including the genera 

 Diplograptus, Dicranoyraptus, Climacograptus, &c.) the common 

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



Fig. 40. Central portion of the colony of Didymograptus divaricatu*, Upper 

 Llandello, 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 

 pristis, showing variations In the append- 

 ages at the base. Upper Llandello, 

 Dnmfreisshire. (Original.) 



Fig. 42. Group of Individuals of Phyllo- 

 grnptus typus, from the Quebec group of 

 Canada. (After Hall.) One of the four rows 

 of cells is hidden on the under surface. 



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



