Dog Lichen, Peltigera sp., grows in damp, badly drained habitats such as mossy logs and rocks but it can also get established on garden lawns. Its leafy lobes have white dog-tooth-shaped root-like rhizines on their undersurface, loosely connecting them to the substrate.
There are several similar species, so I’ve posted my original photograph on iSpot Nature, ispotnature.org, to get a second opinion on the specie: perhaps it’s P. membranacea, rather than P. canina itself? It was taken last April in the marshy woodland beyond Friar’s Crag, Derwent Water.
With those dog-like ‘teeth’ it was once used in an attempt to treat rabies.