John Davidson took this photograph of oak mazegill, sometime in the early 20th century.
Daedalea quercina is an inedible fungus native to North America and Europe. It is found almost exclusively on oaks (the genus Quercus), hence the epithet quercina. Associated with brown rot in the wood of oaks, arborists need to be able to identify this and other types of wood decay fungi to assess urban trees for weakening.
Photography resource link: Conservation Photography - Art Born of Environmental Ethic, an article by Cristina Mittermeier for NatureScapes.net. It relays the story of Peter Dombrovskis, whose photography helped prevent the damming of the Franklin River in Tasmania (visit both the cross-section and posters sections of that site to see examples of this late photographer's work).