Abstract
The positioning of the Siluro-Devonian boundary is reviewed in the light of recent studies in both shelly and graptolitic facies in Europe and elsewhere. A case can be made for retaining the present position at the base of the Ludlow Bone Bed; but international opinion may demand a new standard section in the graptolitic facies, for example in Central Europe. In this event, it is suggested that a position at the base of the Monograptus uniformis Zone might prove the most widely satisfactory solution.

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