Hypholoma fasciculare

Scientific name:  Hypholoma fasciculare (Huds.) P.
Kumm.
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.