There are over 300 species of this conical, spiral-shelled univalve mollusk (see both listings), but few are edible. Periwinkles, also called bigaros, sea snails or winkles, are found attached to rocks, wharves, pilings, etc. in both fresh and sea water. The most common edible periwinkle is found along the Atlantic coasts of Europe and North America. It grows to about 1 inch in size and is gray to dark olive with reddish-brown bands. Periwinkles are popular in Europe but rarely found in the United States. They're usually boiled in their shells, then extracted with a small pick.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.