["Resupinate" fungi are those which are spread out and flattened across their substrate (i.e., what they are growing on), with their spore-producing layer facing outwards.]
This fungus presents a rather unusual appearance, and underwent some dramatic changes over a period of just a few weeks.
The area shown in this photograph measures about five centimetres from top to bottom. The fungus is growing on the bark of a standing dead tree, from which large patches of the bark are missing. As I discovered by carefully comparing its bark with that of living trees in the same street, the host tree is Sorbus intermedia (Swedish Whitebeam), a species which is not native to Britain, but which is often planted here to line streets. As its name suggests, the tree is native to Scandinavia.
The fungus shown here appears to be, at least in this location, strictly a winter species; it was in evidence from December 2007 onwards, completing its life-cycle in about two months. The following December, the same fungus appeared, although in lesser quantities, on the same tree.
I was unable to identify this species myself, and even experts describe the toothed resupinates as a difficult group. However, I was later contacted by someone with considerable expertise in this field (but who declined to be credited). It was stressed that a microscopic examination is essential for an accurate identification, and that working from photographs alone is not to be recommended; however, with those provisos, Hyphodontia quercina (older syn. Grandinia quercina) was suggested as a possibility. That species is apparently widespread but under-recorded. It is also very variable in its macroscopic appearance, and coralloid forms evidently occur; for more information, see: Link