Originally Posted by echinosum

I don't think this is Agaricus. The stem looks rather tough and fibrous for that, and also it has a well-defined hollow section - now Ag stems can become hollow with age, but this doesn't look old. Also shiny top is unusual for Ag. Also it is rather an odd time of year for agaricus to be appearing, though I have occasionally seen some out-of-season ones, often big fat ones on grass verges by roads. Also up between stones is an odd location for an Agaricus, with the exception of A bitorquis the "pavement mushroom", but it isn't that. Immature Agaricus have white gills which become dark brown on maturity. If this is an Agaricus, its gills are surprisingly pale for such a well-expanded cap. Even if it is an Agaricus, never eat one until its gills have darkened because you can't be sure until then.

Something fairly small with a brown cap growing up between stones in May is very unlikely to be of culinary interest. The interesting things that grow at this time of year are mostly St Georges mushrooms and morels.