Tidepool Tim says,  “Periwinkles are a very abundant snail along our coasts and shore.  Sometimes it is possible to find thousands on one large ledge where they are feeding.  These snails graze along on blue-green algae that covers the rocks. Sometimes they are herded up on a large piece of kelp that has washed ashore.  Using their tiny, toothed tongue or radula they slowly lick the kelp to death as they feed on its tissue. in our area, many fishermen / harvesters make good cash money picking periwinkles along the beaches.  Each pound of snails may earn them 50 cents to $1.50 / lb. for their work. Snails are put in mesh sacs and shipped to boston and n.y. for sale in asian markets mostly. The largest periwinkles live at the lowest tide mark - some of these snails approach 2" in size.  Periwinkles get eaten by sea stars and also moon snails which love their tender insides. A close look at these snails reveals a small trap door or operculum that opens and closes when the snail comes out or closes up into its shell. Kids like to hum to periwinkles - this vibration sometimes makes the snail come out of it's shell.”