The De Folin's Lagoon Snail (Caecum armoricum) is a species of marine snail found in Britain.
Discovered in Britain for the first time in 1987, this species is restricted to the Fleet Lagoon in Dorset. Abroad found in France to Portugal, this species has yet to be found elsewhere in Britain. Measuring up to 2mm long, this species has a long thin shell, it can be found between small pebbles. The shell is slightly curved. This species is classed as a Red Data Book species in Britain, though little is known about it.