The Dalradian deformation and metamorphism in western lreland is shown to predate the early Arenigian on the basis of detailed examination of existing and new graptolite collections from Ordovician sediments post-dating the deformation and metamorphism. The faunas show markedly 'Pacific' aspects. Early Ordovician fauna! provinciality into 'Pacific' and 'Atlantic' realms is related to the time of maximum separation of the Laurentian and European forelands by sea-floor spreading in a Proto-Atlantic ocean. It is concluded from a re-assessment of the 'Pacific' and 'Atlantic' fauna! sequences that the North American Canadian Series correlates with the British Tremadocian and lower Arenigian Series.