Littorina arcana
Littorina arcana is a species of sea snail, a marine gastropod mollusk in the family Littorinidae, the winkles or periwinkles.
Littorina arcana has a shell largely indistinguishable from Littorina saxatilis. It breeds by laying egg masses, whereas L. saxatilis produces crawl-away young, so the animals can be distinguished by dissection only.