A yellow gelatinous mass of lobes and folds. Pale forms are occasional.  It becomes more orange and shrivelled if it dries out, but rehydrates.  It is parasitic on Peniophora species of crust fungi that grow on dead deciduous timber, such as birch, hazel and gorse. The host Peniophora is not usually evident because Yellow Brain feeds on the mycelium - so it looks as though it is erupting directly from the wood.

Naematelia aurantia (formerly Tremella aurantia) is a very similar fungus that is parasitic on Stereum hirsutum, the Hairy Curtain Crust - a very common fungus on dead hardwood such as oak or beech.  

Photograph in habitat; note substrate and associated tree species

Appearing on fallen deciduous branches where they have previously been infected with Peniopora, which it paraitises.  The host fungus can sometimes show on the same branch.

All year but especially in autumn

It is almost impossible to tell the difference between Tremella mesenterica and Naematela aurantia without careful microscopic examination - Tremella spores are larger.  If found, the host fungus Stereum hirsutum or Peniophora would identify the parasite, but is not often evident; the Tremella/Naematelia feed on the mycelium under the bark. The national distribution of Naematelia is to the south and south-west (ref. NBN overview for the species) and it we do not believe it has been found in VC55 - but it may have been overlooked or mistaken for Tremella mesenterica. 

There is more information on the excellent 'First Nature' website