TlIK OKDOVICIAN SYSTKM 



111 



successive members of one local series with those of another and 

 with those of other countries. The following is a table of the 

 zones recognised in the British Islands : 



/Zone of Dicelloyraptus ancens. 



complanatus. 

 Pleurograptus liuearis. 

 Dicranograptus Clingani. 

 Climacograptus Wilsoni. 

 Didymograptus superstes. 

 Dicellograptus divaricatus. 

 Didymograptus Murchisoni. 



bifidus. 



hirundo. 



exteusiis. 



Fig. 24. GROUP OK ABENIG AND SKIDDAW CiRAPTOLITES. 



1. Didymograptus hirundo. 



L'. Didyniograptua gibterulus. 4. Phyllograptus typus. 



5. Tetragraptus quadribrachiatus. 



3. Cryptograptus antemiarius. 



4. Phyllograptus t vims. 



The following are the principal species which characterise the 

 several divisions of the Ordovician System : 



Fossils of the Arenig Series 



Hydrozoa. Tetragraptus quadribrachiatus, T. serra, Didymograptus 

 rxtunsus, D. hiruudo, D. gibberulus, Phyllograptus 

 typus, Dendrograptus arbuscula, Diplograptus deutatus, 

 A/ygograptus suecicus. 



