Twenty-nine mudstone samples from coastal exposures of the upper Campanian Northumberland Formation on Hornby Island, British Columbia, Canada, have yielded diverse assemblages of exceptionally well-preserved dinoflagellate cysts belonging to the family Areoligeraceae. In total, at least 11 species belonging to eight genera occur throughout the studied section. Emendations are proposed for the genera Canningia, Canninginopsis, Circulodinium, Cyclonephelium and Senoniasphaera. The new species Canningia diastatikos sp. nov. is proposed. The assemblages include one species complex, which we refer to as the ‘Areoligera volata complex’, that exhibits morphological intergradation between forms typical of Areoligera coronata and Areoligera volata. It is postulated that morphological intergradation may reflect ecophenotypic variability which may present utility for palaeoenvironmental reconstructions.