This small, white capped fungus seems appropriately named as the egghead mottlegill (Panaeolus semiovatus) having a cap that is undoubtedly egg-shaped.
A dependency on the dung of herbivores (especially cows and sheep) means it will usually be found on grazed pasture. It is a simple but effective piece of cyclic ecology; the mycelium of the fungus breaks down the dung and then, when the dung is nearly completely gone, the fruiting body appears and spores are released onto the nearby grass. The cows or sheep then come along grazing on the grass and consume the spores which pass through the animals gut to be ejected as fresh dung for the fungus to start feeding on anew. The size of the fruiting body will depend on the nutrient content of the dung.
It is common and can be seen from spring through until the early winter and it will be seen mainly as a solitary specimen but occasionally you may find a small troop. It is not edible.Panaeolus semiovatus: the egghead mottlegill