Patellaria

The fruit bodies are disc-like (and technically known as apothecia). They are black, stemless and about 2 millimetres in diameter. Often many grow together in close proximity or abutting one another.   

 They are found on dead wood.

 Often it is possible to get a good idea in the field that you are looking at the apothecia of Patellaria, but to be sure you do need to study a specimen. 

 Look-alikes

 Various species of lichenised fungi with small, black apothecia are common on dead wood. In such cases there will be a surrounding lichen thallus, but for some species the colour of the thallus may be faint so that the thallus is hard to pick out from the wood.