102 



HISTORICAL PAL^.ONTOLOGY. 



rent than real in Bohemia, they are exclusively confined to 

 strata of Lower Silurian age, and are not known to occur in 



Fig. 40. — Central portion of the colony oi Didymogra^tus divaricntiis, Upper 

 Llandeilo, Dumfriesshire. (Original.) 



the Upper Silurian. Lastly, there is a group of Graptolites 

 {Phyllograpftis, fig. 42) in which the colony is leaf-like in form, 



Fig. 41. — Examples of Diplograptus 

 fristis, showing variations in the appen- 

 dages at the base. Upper Llandeilo, 

 Dumfriesshire. (Original.) 



Fig. 42. — Group of individuals of Phyllo- 

 grnptus typus, from the Quebec group of 

 Canada. (After Hall.) One of the four rovk^s 

 of cells is hidden on the under surface. 



and is composed oi four rows of cells springing in a cross-like 



