Shell false umbilicate, acutely conical ; whorls 9, planulate, whitish, ornamented with narrow, close, obliquely descending rosy or purple lines, and sculptured with numerous small, inconspicuous, granose spiral lirse ; upper whorls subnodose at the sutures, the lower nearly smooth ; last whorl carinated, a little compressed in the middle, planulate beneath, and ornamented with radiating lines and 8 to 9 concentric lirae; aperture rhomboidal ; columella straight, with 4 or 5 teeth ; basal margin tuberculose within. Alt. 40 mm, diam. 38 mm.

Shell rather large with pointed apex, the profile not (or only very little) stepped, often cyrtoconoid with the last whorl bluntly keeled. Dark with fine mottled pattern and a large white patch in the umbilical area. Columella with distinct tooth; umbilicus minute or closed. The shell is somewhat globose with 5-6 slightly swollen whorls meeting at shallow sutures. The general appearance is smooth but there are fine spiral ridges and prosocline growth lines, sometimes exaggerated and then representing annual growth checks (Williamson & Kendall, 1981). This ornament is clear on young shells but is less, and often eroded, on older ones. The columella is thick and the inner lip folds out over the umbilicus leaving a depression and sometimes a minute hole. Whilst the general colour of the shell is dark the ground colour is buff and this is marked extensively by streaks of brown, green or red running in a prosocline direction. Up to 30 mm high, 25 mm broad; last whorl occupies 75% of the shell height, aperture 35-40%.
The tip of the snout is slightly papillated. the cephalic tentacles gently lobed laterally, each with a broad basal eye stalk connected over the base of the tentacle to a smooth-edged cephalic lappet and, posteriorly, to a neck lobe. The left lobe has 10-12 marginal processes, the right is smooth but has one process on its underside. The foot has an epipodial fold along each side with three tentacles under it, each with two sense organs at its base; a grooved area lies under the operculum. The body is greyish green with many black or purple lines; each tentacle has usually a central dark line with divergent lateral branches. The foot sole and the epipodial sense organs are pale.

P. lineatus is found on rocky shores, preferring areas with rock pools, avoiding those with loose boulders, sometimes with weed, sometimes where the rock seems bare, between L.W.N.T. and H.W.N.T. approximately. The animals browse on algae, mainly microphytes and detritus, rarely on macrophytes. Their distribution in the British Isles is, for unknown reasons, patchy• and they are confined to south west England, the Channel as far east as St Alban's Head (Hawthorne, 1965), the Irish Sea and the south and west coasts of Ireland (McMillan, 1944; Williams. 1965; Nelson-Smith, 1967). Abroad the animals are found on the western Channel coast of France and south to Portugal.

Spawning occurs in summer (May to August) when green eggs are laid, each surrounded by a jelly coat. The larva hatches rapidly and settles after 4-5 days, low on the beach, migrating later to a higher level (Williams, 1965; Desai, 1966; Underwood, 1972; Garwood & Kendall, 1985). The young snails grow rapidly at first, reaching a shell height of about 15 mm at the end of one year and about 22 mm after two, at which time they start to reproduce.