The position of the lower boundary of the Ordovician System has long been in dispute. Interest in this question has been revived following a recent article by Whittington and Williams (1964), who, for historical reasons, site the boundary at the base of the Arenig Series. An attempt is made to justify placing the boundary at the base of the Dictyonema flabelliforme sensu lato Zone of the Tremadoc Series, in the belief that on this occasion there are grounds for allowing ease of application to overrule priority.