I believe it to be a polypore, but was unable to identify the exact type.
Those look like oyster mushrooms. They are the right size and color and have the right type of gills.
Pleurotus ostreatus, is a common edible known for its oyster-shaped cap. One of the first things you should look for when trying to identify this mushroom is the presence of decurrent gills.
Decurrent means that the gills are attached to and run directly down the stem. Take a look at the close up to the right. Can you see how they run into and down the stem?
Oyster Mushroom Identification (Definitely check the pictures out as they look very close to yours.)
For more sources with very close pictures see,
Do note that while oyster mushrooms are edible, eating the wrong mushrooms can kill you so I would take these to a local to be absolutely certain.