Netted crust fungus Byssomerulius corium growing on the underside an oak twig. Note the characteristic pitted surface and curled-over edges. The twig was about 2cm thick.
I prepared a slide of the spores using the same technique I used for Peniophora quercina. The above image (x600) shows the spores to be rounded cylinders, noticably shorter than those of P. quercina but similar in width. This is roughly consistent with the descriptions given at Mycobank which can be summarized by 'cylindrical to subcylindrical to ellipsoid, 4.5-9 x 2.3-4.5 um'.
The original specimen was taken from Whiteknights Park, Reading, UK, on 2012-12-06.