Patina pellucida occurs in two facies with characteristics of habitat and structure, the latter' affecting mainly the shell. One variety, pellucida, which lives on the fronds of Laminaria, has a smooth, elpngated oval and low shell, which is transparent and brown in colour, with 2-8 blue rays radiating backwards from the summit which is placed at the anterior end; it is normally devoid of either epiphytic or epizoic growth. The soft parts are pigmented because of the translucency of the shell, the radular teeth only slightly worn, and the arrangement of the gut coils adapted to a long, narrow haemocoelic space. The second variety, laevis, lives in caves in the holdfasts of Laminaria, and has a rough, round and usually high shell, though the proportions are more variable than in the variety pellucida. The shell is opaque and brown in colour, with 2-46 blue rays and also red-brown ones more or less regularly alternating with them; it has a central summit and is frequently covered with growths of various sorts. The soft parts are not pigmented, the radular teeth are considerably worn, and the coiling of the gut modified to fit into the rounder haemocoel.

The life history may be summarized: the animals breed maximally in winter and spring, the planktonic larvae settling mainly in May as spat about 2 mm. in length. They grow so as to reach a length of about 5 mm. in the following autumn and 10 mm. after a year of sedentary life, becoming sexually mature at 5 mm. length. The majority of the animals die after a settled life of 12 months, but a few linger on into a second year: nearly all these belong to the variety laevis, living a sheltered life within the holdfast of the weed, not exposed like pellucida to the effects of wave action and storm.