This purplish-brown, banded snail is the largest of the three intertidal species of periwinkles. The common periwinkle is a grazing herbivore, preferring the more tender, green algae like Cladophora and Enteromorpha, but is fully capable of grazing on tougher species like Fucus and kelp. They feed using a radula like the rough periwinkle, scraping and raking the rock surfaces.

Common periwinkles are very gregarious, or friendly, often forming dense clusters with several hundreds of individuals per square meter. These periwinkles, because they form such dense populations, are responsible for limiting the growth of green algae and modifying the surrounding environment by keeping them algae free.