Diagnostic characters
Shell apparently smooth, with well developed spire, sharp apex, tumid whorls. Columella with groove leading to umbilicus. Aperture wide. Four orange-brown spiral bands on last whorl. Foot with two tentacles projecting from under operculum.
Other characters
The shell is delicate, covered with a thin periostracum. There are 5-6 whorls which, though tumid, are often a little flattened at the periphery. There is no ornament visible to the naked eye apart from markedly prosocline growth lines, but fine spiral ridges and grooves lie under the periostracum. The aperture is large, the outer lip curved above the periphery, rather straight towards the base where it everts a little. At the base of the columella it curves to form the abapertural edge of a groove leading to the umbilicus, the adapertural edge of which is derived from the inner lip. Shelly material white but appears horn-coloured from the overlying periostracum. The four bands on the last whorl may fuse to give three, two, or a single one; apex of shell homogeneous purple-brown. Up to 10mm high, 5 mm broad; last whorl 80-85% of shell height, aperture 55-60%.
The head has a long, broad snout with terminal mouth. At its base arise two long tentacles each with a basal eye. In males the penis arises behind the right eye and curves back within the mantle cavity. The foot is long, truncated anteriorly and rounded posteriorly. Body yellow, in some animals a little pinkish, with grey on the sides of the foot.
L. vincta has a northern range, the British Isles being near its southern limit. During most of the year the snails live sublittorally, but they migrate upshore to breed and then occur commonly, sometimes abundantly, at about L.W.S.T. on all British and Irish shores where there is a good growth of fticoids, particularly Fucus serratus, or of red weeds, on which they feed preferring sporelings (Fretter & Manly, 1977b). The animals are often very destructive. They breed over a long period in spring, laying between 1000 and 1500 capsules, each with a single egg. The capsules are embedded in jelly and laid on weed, mainly F. serratus, the whole spawn forming a ring or short spiral about 3 mm in diameter. The young hatch as veliger larvae which settle 2-3 months later. The animals are annuals, dying after spawning.