The trilobite fauna of the basal Didymograptus superstes Mudstones (Nemagraptus gracilis zone) at Aldons Quarry is described. The fauna is large, consisting of 68 species, of which 12 are new, belonging to 47 genera. The fauna bears a much closer resemblance to that of the Albany mudstones than to that of the platy upper Stinchar Limestone: this is attributed to environmental control. The strong North American stamp of Barr Group faunas is at a maximum: Stenoblepharum, Raymondella, Chomatopyge and Miraspis appear for the first time in the Girvan District.