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Since the publication of the Cambrian and Ordovician correlation charts by the Geological Society (Cowie et al. 1972; Williams et al. 1972) it has become more general practice to include the Tremadoc Series in the Ordovician, and although the precise level at which the base of the Ordovician is to be drawn is not finally decided, the limits of the system are generally understood. The upper limit has now been precisely defined at the base of Parakidograptus acuminatus Biozone, as exposed at Dob's Linn in the central belt of the Southern Uplands in south Scotland, the underlying Glyptograptus persculptus Biozone being relegated to the Hirnantian Stage of the Ashgill Series in the Ordovician System (Cocks & Rickards 1988).
Standard international series for the Ordovician are yet to be decided upon but in Britain the series usually employed are those which have been defined and refined over a period of more than a hundred years, based on successions in Wales, the Welsh Borderland and the north of England, namely: Tremadoc Arenig, Llanvirn, Llandeilo, Caradoc and Ashgill. These have ready application in the Anglo-Welsh area and further afield, particularly in continental Europe and Asia, but the demonstration that the faunas of Scotland and northwestern Ireland have much more in common with those of Laurentia (North America, Greenland and Spitsbergen) makes the direct application of many of these chronostratigraphical terms less relevant than the scheme which has developed for the classification of the North American Ordovician. The latter is still to some extent
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