Coprinus atramentarius

Scientific name:  Coprinus atramentarius (Bulliard:Fries)
Fries
Derivation of name: Atrament- means "ink" in reference to
the deliquescing gills. 
Synonyms: Coprinopsis atramentaria (Bull.:Fr.) Redhead,
Vigalys & Montcalvo. This is actually the most recent name for
this mushroom, reflecting the division of the genus Coprinus
into four genera. The Miller field guide (2006) for example,
uses this name.
Common name(s):  Alcohol inky
Phylum:   Basidiomycota
Order:   Agaricales
Family:   Coprinaceae
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.

More information at MushroomExpert.com:  


Figure 1. A clump of the alcohol inky. Note the location of
the inferior ring. Photo © William Roody.

Figure 2. A clump of alcohol inky. Grooves or striations
occur near the cap margin. Photo © Steve Nelsen.

Figure 3. The gills of the most mature specimens are starting
to turn into a black inky fluid. Photo © Pam Kaminski.