Pleurotus ostreatus

Scientific name:  Pleurotus ostreatus Fries
Derivation of name:  Ostre- means "oyster" and atus
means "resembling."
Synonyms:  
Common name(s):  Oyster mushroom.
Phylum:   Basidiomycota
Order:   Agaricales
Family:   Pleurotaceae
Occurrence on wood substrate:  Saprobic or parasitic;
solitary to more typically in overlapping clusters on living or
dead deciduous trees, on decaying logs and stumps,
sometimes on conifers; April through November, year-round
during mild periods.  
Dimensions:  Caps 5-20 or more cm wide; stipes 0.5-4 cm
long and 0.5- 3.5 cm thick. Stipes may be absent.   
Cap:  Moist or dry; smooth; variable in color: whitish to
cream, grayish to brown, some with lilac tones; oyster shell-
shaped to fan-shaped or semicircular.
Gills: Decurrent or radiating from point of attachment;
whitish, yellowish in age.
Spore print: White to pale lilac-gray.
Stipe: Sometimes absent or rudimentary. If present, lateral
to eccentric or even central if fruitbodies are on top of a log
or stump; whitish; hairy at base.
Veil: Absent.
Edibility: Edible, rated as choice.
Comments: The variable forms of this mushroom are usually
accounted for by describing this fungus as a species
complex. The web sites below contain additional information
about this and links to evidence that there are at least three
closely related species which have all at one time or another
been called Pleurotus ostreatus.

More information at MushroomExpert.com:   
More information at TomVolkFungi.net:

 
Figure 1. Typical occurence of oyster mushroom.
Photo © Gary Emberger.

Figure 2 Oyster mushroom typically grows in overlapping
clusters. Photo © David Work.

Figure 3. Decurrent gills. Photo © David Work.

Figure 4. Oyster mushroom on a stump. Logs are another
common habitat. Photo © William Roody.

Figure 5. Smooth, overlapping caps. Photo © Gary
Emberger.

Figure 6. Oyster mushrooms with brownish/violet coloration.
Photo © Pam Kaminski.

Figure 7. Although these oysters appear to be growing in
grass, they are actually growing on buried wood. A large
pin oak tree was removed from this site just a few years
before the picture was taken. Photo © Gary Emberger.


Figure 8. The oysters of Figure 7. These were growing in
January during a period of exceptionally mild weather. Late
season oysters often are tan or gray in color. In addition,
these caps produced a distinctly gray-lilac spore print.
Photo © Gary Emberger.

Figure 9. A yellowish, commercially cultivated variety of the
oyster mushroom. Photo © Gary Emberger.