I saw these periwinkles in a tide pool on the coast of Maine recently. I read that they are a small edible species of marine gastropod in the family Littorinidae, the winkles. They were introduced to the Atlantic coast, possibly by rock ballast in the mid-1800's. Unfortunately, their introduction damaged the native gastropod population. But they do add texture to this already very textural photo I took in Acadia National Park.