Hypholoma fasciculare

Scientific name:  Hypholoma fasciculare (Hudson:Fries)
Kummer
Derivation of name: Fascicul- means "bundle" referring to
the clustered or bundled growth habit of this species. 
Synonyms:  Naematoloma fasciculare (Huds.) P. Karst.
Common name(s):  Sulphur tuft
Phylum:   Basidiomycota
Order:   Agaricales
Family:   Strophariaceae
Occurrence on wood substrate: Saprobic; in clusters on
deciduous or conifer logs, stumps, or surrounding soil from
buried wood; May through November.   
Dimensions:  Caps are 2-8 cm wide; stipes are 5-12 cm long
and 1.5-10 mm thick. 
Cap: Smooth; moist or dry; orange-yellow to sulphur-yellow
to greenish-yellow.        
Gills: Attached; sulphur-yellow to greenish yellow, becoming
pale purple-brown at maturity.
Spore print: Purple-brown.
Stipe: Yellowish.
Veil: Partial veil leaves a fibrous zone near the stalk apex
which may become purple-brown following deposition of
spores.
Edibility: Poisonous.
Comments: Causes gastric distress.

More information at MushroomExpert.com:   

Figure 1. Clusters of sulphur tufts on a log.
Photo © Larry Grand.
    

Figure 2. The gills on these young specimens are still
yellowish. They will become darker as the spores mature.
Photo © William Roody.

Figure 3. The lower portions of the stipes are typically darker.
Photo © John Plischke III.