Calycina citrina

Scientific name: Calycina citrina (Hedw.) Gray
Derivation of name: Citrin- means "lemon-yellow."  
Synonyms: Bisporella citrina (Batsch) Korf &
S. E. Carp., Helotium citrinum (Hedw.) Fr., Peziza
citrina Batsch. 
Common name(s):  Yellow fairy cups
Phylum:   Ascomycota
Order:   Helotiales
Family:   Pezizellaceae
Occurrence on wood substrate: Saprobic; in clusters on
decaying wood; summer through fall.  
Dimensions: Cups are 1-3 mm wide; stalkless or with
thick, stalk-like base.            
Sterile outer surface:  Bright lemon-yellow to golden-
yellow; smooth.
Fertile inner surface: Same coloration as sterile surface;
smooth.
Comments: Although individually tiny, these fungi often
occur in such large numbers that they become quite
conspicuous.

More information at MushroomExpert.com:   


Figure 1. The bright yellow fruit bodies of Calycina
citrina are very common on decaying wood.
Photo © Gary Emberger.


Figure 2. The color of the fruit bodies ranges
from bright lemon-yellow to golden-yellow.
Photo © Larry Grand.


Figure 3. Calycina citrina may have a
rudimentary stalk, as shown here, or it may be
stalkless. Photo © Gary Emberger.

Figure 4. Yellow fairy cups typically occurs in groups or
dense clusters. Photo © Gary Emberger.

Figure 5. The fertile inner (upper) surface and outer
surfaces are the same color. Photo © Gary Emberger.


Figure 6. Another small cup fungus, Chlorosplenium
chlora, has a bright yellow outer surface but the fertile
surface becomes greenish at maturity. Calycina citrina
is on the right and Chlorosplenium chlora is on the left.
Photo © Gary Emberger.