Here's how to prepare them, from cooks.com:
1/2 c. salted water
1/4 tsp. thyme
1 clove garlic, mashed
1/2 c. beef bouillon or dry white wine
4 dozen well-scrubbed periwinkles
4 tbsp. butter
Rinse the periwinkles thoroughly; put all ingredients in pot and bring to a boil; add periwinkles and steam until they can be easily extracted from the shell, about 4 minutes. Serve with melted butter.