Arion fuscus, aka "Dusky Arion", is a species of small air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Arionidae, the roundback slugs.Arion subfuscus (Draparnaud, 1805) is considered by some authors to be a partial synonym for Arion fuscus.Arion subfuscus (Draparnaud, 1805) and A. fuscus (Müller, 1774) have very similar morphology and have overlapping distributional ranges in northwestern Europe.