Coprinopsis atramentaria

Scientific name:  Coprinopsis atramentaria (Bull.:Fr.) Redhead,
Vigalys & Montcalvo.
Derivation of name: Copr- means "dung" and opsis means
"resembling."  Atrament- means "ink" in reference to
the deliquescing gills. 
Synonyms: Coprinus atramentarius (Bulliard:Fries)
Fries
Common name(s):  Alcohol inky
Phylum:   Basidiomycota
Order:   Agaricales
Family:   Psathyrellaceae
Occurrence on wood substrate: Saprobic; clustered in grass,
on decaying wood or on the ground from buried wood; May
through September.   
Dimensions: Caps are 5-7.5 cm wide; stipes are 4-15 cm
long and 1-2 cm thick.   
Cap: Dry, gray to gray-brown; with shallow grooves on the
margin (radially lined or striate). Small scales may form near
the center.        
Gills:  Free; white when young, becoming black and inky at
maturity.
Spore print: Black.
Stipe: White, hollow, with a white annular zone near the base.
Veil: Evanescent, leaving a fibrous ring.
Edibility: Edible with caution.
Comments: Alcoholic beverages should not be consumed up
to 48 hours before or after eating this mushroom. The "alcohol"
inky causes coprine poisoning which produces symptoms of
nausea, vomiting, flushing, rapid breathing, and severe headache
This species used to be in the genus Coprinus
but DNA studies (see Redhead, S. A., et al. 2001) radically
revised the taxonomy of that genus: Coprinus was retained
for a small number of species, several new genera were
created, and the members of the family Coprinaceae
were split between two families - Psathyrellaceae and
Agaricaceae.
.
More information at TomVolkFungi.net:
More information at MushroomExpert.com:  

Figure 1. A cluster of Coprinopsis atramentaria mushrooms
at the base of a tree.
Photo © Maynard Wheeler.

Figure 2. Grooves or striations occur near the cap margin.
Photo © Steve Nelsen.

Figure 3. Note the location of the ring, far down on the
stipe. Evidence of a distinct ring diminishes with age.
Photo © William Roody.

Figure 4. A longitudinal section reveals the hollow stipe.
Starting with the lower edge of the white gills, the gills turn
dark as the spores mature.
Photo © Steve Nelsen.

Figure 5. Mature gills are black.
Photo © Maynard Wheeler.

Figure 6. The gill edges appear eroded, the result of the gills
auto-digesting (deliquescence), becoming black and inky. The
process strats at the cap margin and works its way upward.  
Photo © Maynard Wheeler.