Pleurocybella porrigens

Scientific name:  Pleurocybella porrigens (Persoon:Fries)
Singer
Derivation of name:  Porrigens means "extending forward,
projecting horizontally."
Synonyms:  Pleurotus porrigens
Common name(s): Angel's wings.  
Phylum:   Basidiomycota
Order:   Agaricales
Family:   Tricholomataceae
Occurrence on wood substrate: Saprobic; solitary to
clustered on dead conifer logs and stumps, particularly eastern
hemlock (Tsuga canadensis); August through October.   
Dimensions:  Caps 2.5-10 cm wide.   
Cap:  Smooth; white; flesh very thin.      
Gills: White; crowded and narrow; radiating from stub-like
point of attachment.
Spore print: White.
Stipe: Absent.
Veil: Absent.
Edibility: Inedible. Miller reports that although considered
edible by many, there are reports of deadly poisonings in
Asia.
Comments: Pleurotus ostreatus, while resembling this
mushroom, is a much more robust species with thicker flesh.
Also, Pleurotus ostreatus occurs most often on deciduous
wood.

More information at RogersMushrooms.com:   

    
Figure 1. Angel's wings. Photo © John Plischke III.

Figure 2. The thin flesh of Angel's wings is readily apparent
in this photograph. Photo © Steve Nelsen.

Figure 3. Pleurocybella porrigens. Photo © Steve Nelsen.