TRILO BITES AND OTHER FOSSIL CRUSTACEA. 189 



geological systems. Thus, Paradoxides and Agnostus 

 are peculiarly Cambrian ; Trinucleus and Asaphus 

 are almost exclusively Lower Silurian ; Phacops and 

 Calymene are markedly Upper Silurian ; Brontes and 

 Harpes are among characteristic Devonian fossils ; 

 whilst Phillipsia and Griffithsides are genera of small 



Fig. 152. Paradoxides Davidis. Fig. 153. Homalanotus. 



Trilobites the last of their race which are peculiar 

 to the Carboniferous limestones. 



Undoubtedly many of the fossil Tribolites we 

 meet with in any of the above rocks are moults that 

 is, portions of the carapace thrown off after the 

 manner of the shells of lobsters and crabs. This 



