False Puffball (Reticularia lycoperdon)
Once thought of as a curious form of fungi, slime moulds are now acknowledged as being entirely unrelated. Opinions still differ as to their precise taxonomic classification but they are currently placed in the Protozoa. Although many species are very colourful and quite striking in appearance, they are typically ephemeral and seldom remain in good condition for more than a day or so.
Reminding me of something from a 1950s B-movie (though it probably won't eat you alive), R. lycoperdon is one of the more obvious species. It is typically seen in its reproductive stage as an ivory-white swelling on standing dead trees or on large pieces of fallen timber. The above images show a selection of specimens from this season.
Sterry, P. and Hughes. B. (2009). Collins Complete Guide to British Mushrooms and Toadstools. London: HarperCollins, p. 334.