In addition to Common Periwinkles, I've eaten another local saltwater snail this year, the Atlantic Dogwinkle (Nucella lapillus). Also known as Dogwhelks, these native, pointy-shelled mollusks live in rocky, intertidal zones, alongside the non-native and often more numerous Common Periwinkles. Atlantic Dogwinkles come in a variety of colors (often white), have pointed spires (unlike the blunt spires of Common Periwinkles), and have a groove at the base of the shell opening.
Scouting out a low-tide Biddeford location, I was able to find areas with more Dogwinkles than Periwinkles, enough for me to feel comfortable harvesting a dozen to eat. After boiling them for a minute or two in saltwater, extracting them with a pin, and removing each operculum (the thin cover on their shell-opening), I enjoyed them as an oceany appetizer.
The top photo shows both snails -- Atlantic Dogwinkles (with white, pink, yellow and black shells) and Common Periwinkles (a light-tan one on left and a smaller, darker one on right). To view the following images in full-size, click here.