102 



HISTORICAL PALAEONTOLOGY. 



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 of Didyntrgrnptns divaricntus, Upper 

 Llandeilo, Dumfriesshire. (Original.) 



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

 (Phyllograptus, fig. 42) in which the colony is leaf-like in form, 



Fig. 41. Examples of Diplografitus Fig. 42. Group of individuals of Pkyllo- 



fristis, showing variations in the appen- gr.iptus typus, from the Quebec group of 



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



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



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



