The British-Scandinavian and North American areas lay within different zoogeographical provinces in Lower Ordovician times, a fact best demonstrated by differences in shelly faunas of this age, but reflected also, to a certain degree, in the distribution of planktonic graptoloids. Provincialism among the graptoloids is evidenced in particular by the greater diversification and time-range of pendent forms of Didymograptus in northwestern Europe in comparison with North America; however, other elements of the Lower Ordovician graptoloid faunas—such as the biserial scandent forms—show a comparable development in the two areas and provide a firm basis for correlation. The correlation proposed below allows an acceptable equation of shelly faunas between the two provinces.

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