THE STORY OF THE EARTH. 



spines upon all parts of the body. As the 

 Trilobite grows it is said to shed its shell like 

 other Crustacea; and with this growth there come 



<• additional plates added to the abdomen, so 

 that there is a sense in which the ancient types, 

 like Pa nay be regarded as the most 



complex! 



Among the Lower Cambrian genera are Para- 

 , DikelocephaluSy Olcnus, Conocoryphe. In 



the middle and Upper Cambrian or Ordovician 



rocks, the common genera include Ogygia, Asa- 

 ph us, Tri nucleus, Lichas, Andaspis. PhaCOps 

 ranges through the Silurian and Devonian; and 



llhenus ranges from the Upper Cambrian to the 

 Silurian. 



In the Silurian, Calymene^ Encrinurus y Phacops % 

 and Homalonotus are characteristic gen< 



In the Devonian rocks, PhaCOps, Homalonotus 



and Bronteus are commonest. 



In the carboniferous, Phtilipsia is the best- 

 known genus. 



CHAPTER XII. 



oil. ki I) SANDSTONl w !» DEVONIAN PERIOD. 



A OR! \i unconformity is inferred to divide 

 - man rocks below from the overl\ 



\ i >rth of the Bristol Channel there is no 

 evidence of marine origin for the deposits which 

 appear to have been accumulated in great lakes 

 land sui I South of the Bristol Chan- 



el through and ( rerm; 



les which follow upon the Silurian 

 marine. Thej emarkable for the 



