In the field I'd write Byssomerulius corium on the bag in which I popped the specimen, but I'd like to look at the spores later. As KenT says if this were a brown/orange colour something like Serpula could be a candidate, especially if it were dark brown centrally (where the most mature spores would be) and paler outward (where the spores have not yet acquired their deep colour). This photo (http://www.cpbr.gov.au/fungi/images-captions/serpula-lacrmans-0159.html) shows areas with mature and immature spores. Given the well-developed ridges, still whitish, in KenT's specimens, Serpula and the like are unlikely. Byssomerulius corium is quite common and may stay white or creamy for a long time or become orange (except possibly for a whitish margin. The fungus may remain flat but, especially in larger specimens, it is common to see the margins bending out a bit.