HISTORY OF RESEARCH. xcvii 



Rhabdopliora in time necessarily gives to the collective Graptolitic fauna of each 

 of the subsystems or major formations of the Lower Palaeozoic rocks a special and 

 distinctive aspect that renders it capable of immediate identification all over the 

 world. The Arenig division is recognisable at a glance by its crowds of Phyllo- 

 graptidse and Dichograptidas ; the Bala, by the absence of these families and the 

 presence of multitudes of Dicellograpta and Diplograpta ; the Valentian, by the 

 absence of the former and the presence of the latter in association with Mono- 

 graptidae ; and the higher Silurians by the absence of the Diplograptidae and the 

 presence of Monograptidae alone." 



(8) " The further restriction in time and vertical extension of the species and 

 varieties of the Rhabdopliora places in our hands the material available for a more 

 minute subdivision of the formations of the Lower Palaeozoic rocks than has 

 hitherto been attempted. These subdivisions, or Graptolite horizons, answer 

 roughly to the Ammonite zones of the Jurassic rocks of Europe, and will, in all 

 probability, prove of equal value in the correlation of widely separated deposits." 

 At present the following zones are recognisable, many of them of extraordinary 

 geographical range : 



Upper Cambrian. 



1. Zone of Bryograptus Gallavei. 



Lower Ordovician. 



2. Zone of Tetragraptus bryonoides. 



3. Zone of Didymograptus bifidus. 



4. Zone of Didymograjptus Murohisoni. 



Upper Ordovician. 



5. Zone of Gcenograptus gracilis. 



6. Zone of Dicranograptus Glingani. 



7. Zone of Pleurograplus linearis. 



8. Zone of Dicellographis complanatus. 



9. Zone of Biccllograptus anceps. 



Silurian System. 



10. Zone of Diplograptus acuminatus. 



11. Zone of Diplograptus vesiculosus. 



12. Zone of Monograptus gregarius. 



13. Zone of Monograptus spinigerus. 



14. Zone of Rastrites maximus. 



15. Zone of Monograptus cviguus. 



16. Zone of Gyrtograptus Grayse. 



