TRILOBITES OF THE SILURIAN SYSTEM. 



649 



together with the Agnostus and Ogygia of Brongniart. These, with two or three species 

 of Trinucleus, are confined to the lower beds of the Silurian System. 



The Cambrian Rocks of the region illustrated in this work, are, as far as I know, 

 poor in Trilobites, the only traces of them yet observed being in the limestone near 

 Bala 1 . 



Examining these fossils in the order in which they occur in their native beds, we 

 shall perceive, that as each formation is characterized by peculiar species, so these 

 crustaceans are of great value in determining the age of the deposits ; for though most 

 of the mollusks and conchifers also differ in the successive formations, still a few species 

 of them range almost from the top to the bottom of the Silurian System, whilst no ex- 

 ample is yet known, of a species of trilobite which is common in the Upper Silurian Rocks, 

 being found in the lower beds of the system. I shall describe all the Silurian Trilobites 

 with which I am acquainted in this chapter, beginning with those of the uppermost 

 strata, and terminating with the species peculiar to the lowest. In the mean time I 

 offer a list of the genera and species. 



1 I have seen Trilobites in Devonshire, some of which differ in species from any described in this work. 

 When the older or Protozoic Rocks shall have been thoroughly examined in all parts of the British Isles, it is 

 probable that the list of Trilobites will be materially enlarged. 



