Shell small, up to 24 mm. (1 inch) in length, usually rather thin, ovate, high-arched, the apex immersed and situated towards the anterior end. Surface smooth and polished, with very weak radial riblets. Colour golden brown, black tipped at the apex, and usually with a few-vivid sky-blue lines running back from the apex to the posterior margin. Length 14-24 mm; Width 11-20 mm; Height: 5-10 mm.

Seas of western Europe, from Lofoten Islands, Norway, to Portugal.

Shell cap-shaped or conical, without marginal slit, apical hole or internal septum. Outer surface smooth. Shell either translucent (var. pellucidum) when it has an anteriorly placed apex, or opaque (var. laevis) when the apex is nearly central. Apical area always with some radiating kingfisher blue lines or rays; some red ones may also occur, usually near the margin. Ctenidium absent, but the mantle edge bears pallial gills except anteriorly. Glandular streak on each side of foot anteriorly; no operculum. This limpet is typically found on plants of Laminaria, It exists in two forms: H. p. pellucidum and H. p. laevis, the former on the fronds and stipes, the latter in the holdfast under the stipe. Limpets may also occur on fucoids when adult and are regularly found on them when young, migrating to laminarians when their radula and buccal muscles have grown powerful enough to eat them (shell length about 4 mm) (Vahl, 1971). The shell of var. pellucidum (alive or fresh) is thin, horn-coloured, with a dark area parallel to the apertural edge and another at the apex. The lips of the aperture lie in one plane. It grows to a length of about 14mm. In var. laevis the shell is nearly conical and thicker, with a rougher surface; the apertural lip does not lie in a plane; its apical area consists of a small shell with the characters of pellucidum. Blue rays are rare except in this area (Graham & Fretter, 1947). It measures up to 18mm long. The animal has a large head with two tentacles, each bearing an eye. The mantle skirt is edged with tentacles; the foot is large. Body white.

Blue-rayed limpets are found on all shores of the British Isles and on all Atlantic coasts between Portugal and northern Norway.

They breed maximally in winter and spring, the eggs being planktonic. The larvae settle in May with a shell 2 mm long, usually on fucoids (Vahl, 1971). Initially all spat are identical in appearance, like Helcion pellucidum pellucidum, later, when the shell is about 4 mm in length, the animals migrate to Laminaria plants, the tough tissues of which they have not so far been able to eat. Those juveniles which take up their position on the fronds retain the pellucidum characters of the shell whereas those that enter holdfasts acquire the features characteristic of H. p. laevis. The adult shell of the latter shows the features of pellucidum in its apical area developed before the mi¬gration to Laminaria occurred.