THE ZONAL RANGE OF THE BRITISH GRAPTOLOIDEA. 515 



In tlie second of the Tables here presented — Table B, " Vertical Range of the 

 Zones of British Graptoloidea," — the stratigraphical nomenclature employed in 

 the Memoir is also given in brief. A comparison of this second table with the 

 tables and details published in the Memoir of 1879-1880, shows how fully the 

 progress of discovery has borne out the original anticipation that •' future 

 research will soon fix more definitely the composition and limit of the characteristic 

 (Graptolite) faunas of the zones (then) already recognised, extending the range 

 of some of the forms into neighbouring ones, and adding largely to the number of 

 zones themselves." 



It must be carefully borne in mind that a Graptolite zone is characterised by 

 a special association of Graptolites, and that that form in this association which 

 apparently combines restricted vertical range with wide horizontal distribution is 

 most conveniently selected as the iiuJex of the zone. 



It is not pretended that each of the Graptolite zones is of equal geological 

 importance. Each zone, however, marks a special stage or horizon in the 

 ascending series of the Lower Palaeozoic rocks. In other words, the Graptolite 

 zones are comparable more or less with the Ammonite zones of the Mesozoic 

 formations. As such their importance as indices of chronological sequence can 

 hardly be over-estimated ; but to what extent they will prove of value as indices 

 of chronological duration, only time and future geological research and discovery 

 can be expected to show. 



