Ganoderma

The fruit body is a polypore. The species may be stemmed or stemless (and then grow bracket-like or shelf-like from wood). The brackets may grow outward to over 20 centimetres and be 10 centimetres thick at the wood surface. The fruit body is very hard and, in larger examples, will wave an uneven upper surface with radial or concentric ridges. The upper surface is smooth and some shade of brown an dmay be shiny (as if lacquered) or matt. The lower surface is composed of numerous tiny pores (you may need a hand lens to see this) and is white or creamy when freshly developed but later brown.      

The spores are brown and very often, especially if the fruit body is say at least dinner-plate size, you are likely to see a brown spore deposit below or near the fruit body.