Any idea which Agaricus?

Any idea which Agaricus?

Observation - Any idea which Agaricus? - UK and Ireland. Description: Convex pale beige cap with flattish scaly top and some splits at sides 6 cm across, white stem 6 x 1.5 cm with some yellowing and fibrous knob at base, tight ring towards top of stem attached at top edge and rolling up, pinky brown free gills, dark brown

Convex pale beige cap with flattish scaly top and some splits at sides 6 cm across, white stem 6 x 1.5 cm with some yellowing and fibrous knob at base, tight ring towards top of stem attached at top edge and rolling up, pinky brown free gills, dark brown spore print. growing in grass at base of young False Acacia tree.