The type section of the British Ordovician, which was not defined originally to include the Tremadoc, contains the standard succession of the Arenig Series and correlatives of part or all of the standard successions of the other series. The latter, except for the Llanvirn, which was proposed for a sequence of graptolitic shale, are based on successions containing shelly faunas. Difficulties remain in interrelating series and correlating them with the standard graptolitic zones, but the current arrangement is practicable.

Shelly faunas of the type successions belong to two stratigraphically successive provinces, a pre-Ashgillian Anglo-Welsh Province and an Ashgillian North European Province, which extended from Quebec to the Baltic and included all of Britain and Ireland. Brachiopods of the type successions can be assigned to an Arenigian-lower Llanvirnian orthid-obolid fauna and an upper Llanvirnian-Ashgillian dalmanellid-sowerbyellid-strophomenid fauna; trilobites can be assigned to an Arenigian-Caradocian trinucleid-asaphid-calymenid fauna and an Ashgillian encrinurid-illaenid-odontopleurid fauna. Pre-Ashgillian shelly faunas of Scotland and western Ireland have a significantly different generic composition, as do pre-Longvillian faunas of Anglesey and southeast Ireland. Cluster analyses of these fa nas and contemporaneous assemblages from North America and Europe show that the faunas of Scotland and western Ireland belong to an American Province and those of Anglesey and southeast Ireland belong to a Baltic Province, and that the former became greatly reduced and the latter were assimilated into the North European Province by the end of the Ordovician.

Maintenance of three independent provincial faunas up to Longvillian time is attributed to oceanic currents flowing parallel with the long axis of the British and Irish segment of the Caledonian geosyncline; a northern American current flowed northeastward toward western Ireland and Scotland; a middle Baltic current moved westward north of the English Midland platform toward southeast Ireland; and a southern Anglo-Welsh current drifted northeastward along the western margin of the English Midland platform. Emergence of the North European Province by Ashgillian time was related to the effects of volcanism and uplift.

Revision of Ordovician stratigraphic nomenclature should include the establishment of standard series for each shelly province identified and should allow for hybrid classifications for regions where provincial boundaries overlapped in time.