I thought the main distinction between amanita (possibly deadly) and
lepiota (parasol, generally edible) was that the amanita has a 'volva'
at the base of the stem - remnant of the 'universal veil' that covered
the egg-shaped young mushroom down to the ground. Lepiota have a ring
of distinctive cross-section, but no volva.
In Europe eating parasol-looking mushrooms with a ring, creamy gills,
and sandy or shaggy cap surface would seem to be OK. In N America you
have the toxic shaggy parasol species with green spores, which can be
detected by putting the cap on a clean sheet of paper for a few hours.
I would steer clear of *any* pure white one, with or without ring
(unless on supermarket shelves).