Like with other Arion species, the pneumostome lies in the anterior half of the mantle, and the back is round in cross-section rather than with a keel. However, the subgenus Carinarion is distinguished by a row of paler tubercles along the midline of the back, giving the impression of a keel; this is less prominent in adults. Another feature of Carinarion is that the cross-section is bell-shaped, with splayed-out rather than inward-curving sides. The overall colour of A. circumscriptus is a monochrome dark grey (sometimes with a tinge of blue or brown), with a darker dorsal band and a darker lateral band along each side, the one on the right running above the pneumostome. The flanks below are paler. The sole is white or pale grey, and the mucus colourless, or sometimes pale yellow. Length reaches 40 mm, or some say up to 50 mm.
