Looks like Omphalotus nidiformis to me. Do the gills emit a very faint light under very low lighting conditions? edit: from wikipedia "Similar in appearance to the common edible oyster mushroom, it was previously considered a member of the same genus, Pleurotus, and described under the former names Pleurotus nidiformis or Pleurotus lampas. Unlike oyster mushrooms, O. nidiformis is poisonous; while not lethal, its consumption leads to severe cramps and vomiting." The main thing that makes me think that is the blackening on top of the cap. I'm not sure about IDing oysters, but i've seen tonnes of Omphalotus'.