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DescriptionThis species has a thickly calcified, glossy shell, with closely spaced transverse ridges. The shell is up to 12 mm long by 8mm wide. The upper surface is reddish-brown in colour with 3 diagnostic spots.The underside of the shell is flattened and white. The head, tentacles, foot and mantle are brightly coloured, from yellow, red, orange and brown, the foot often being paler. When the animal is active the mantle almost completely obscures the shell, and the mantle edge draws out anteriorly into a long siphon. The shell aperture is narrow, running the whole length of the shell, turned left at the ends with both sides ridged.Trivia monacha can be confused with Trivia arctica, the former being larger and having 3 distinctive brown spots on its shell. There is no operculum present, and the females have an additional ventral pedal gland. This species feeds on the ascidians Botryllus schlosseri, Botrylloides leachi, and Diplosoma listerianum. Breeding occurs in late spring and summer. The female deposits flask-shaped egg capsules (each containing 800 eggs), in holes that have been eaten out of the ascidian. After a few weeks the larva emerge and are free swimmming for a few months. Juveniles (5 mm long) have a short spire, wide aperture and lack the characteristic ridging of mature adults.