These mushrooms belong to the family Agaricaceae. The Agaricus genus which contain about 400 species is found throughout the world and include both edible as well as poisonous species. One of the most consumed species of mushrooms in the world the Button Mushrooms Agaricus bisporus is a member of this genus.
In wilpattu I have observed these mushrooms at Kokkare Villu and between Wilachchiya to the Weewala Bungalow during the months of May and November.