This is likely to be an Agaricus, possibly A. arvensis (Horse Mushroom); but more information is needed. Compare it to the description in E-Flora BC.
You could also check Agaricus augustus (Prince's mushroom) which, like A. arvensis, is known in this area.
Hmm... probably not either: A. proxima has a white spore print according to the Wikipedia, whereas my specimen was brown, and A. ovoidea has a red/russet volva, but mine was white.
Wow such a huge Agaricus! With this genus, in addition to your excellent diagnostic photos (& habitat notation), I suggest noting down any odour, and any staining (red or yellow), or absence of staining, that you notice when scratching the stipe. In some cases the appearance of the ring, especially in the younger ones, can be helpful.
