Pleurotus dryinus
Also called Veiled Oyster
White growing on a tree trunk
Has decurrent gills and veil remnants

It is the partial veil (rather than a universal veil of the kind that leaves a volva at the stem base of Amanita and Volvariella species) from which this edible mushroom gets its common name. Fragments of the partial veil can often be seen hanging from the inrolled cap margins of young fruitbodies of the Veiled Oyster .
From first-nature.com Click here for more info and pictures