Myxarium nucleatum is a species of fungus in the family Hyaloriaceae. In the UK, it has been given the recommended English name of crystal brain. Basidiocarps (fruit bodies) are watery white, pustular or lobed, and gelatinous with small, white, mineral inclusions visible to the naked eye. It is a common, wood-rotting species in Europe, typically growing on dead attached or fallen branches of broadleaf trees. It is currently not clear whether collections from North America (where it is called granular jelly roll) and elsewhere represent the same species. Taxonomy History The species was originally described by Karl Friedrich Wilhelm Wallroth in 1833, who found it growing on hawthorn in Germany. He placed it in his new genus Myxarium based on its visible white inclusions which he interpreted as spores. Wallroth noted its similarity to Tremella nucleata, but nonetheless described Myxarium nucleatum as a new and separate species, using the same epithet. Lewis David von Schweinitz had previously described Tremella nucleata from the United States, a species that was later transferred to the genus Exidia by Burt. Many subsequent publications, summarized by Donk (1966) and Reid (1970), discussed whether the European Myxarium nucleatum is conspecific with the American Exidia nucleata. The consensus, based on morphological similarity, was that they are the same. Current status Molecular research, based on cladistic analysis of DNA sequences, has indicated that Myxarium nucleatum belongs in a clade that is quite separate from the Exidia clade. Furthermore, at least three similar species of Myxarium occur in northern Europe, with a single American collection appearing distinct. No DNA-based research has yet been published on the status of "Myxarium nucleatum" worldwide. Description Myxarium nucleatum forms scattered, pustular, gelatinous fruit bodies in diameter. These often coalesce, forming compound fruit bodies that may be irregularly cerebriform (brain-like) and up to across. Fruit bodies are hyaline (colourless) to whitish, occasionally with yellowish tints. Opaque, white, spherical, mineral inclusions are visible with the naked eye and are made of calcium oxalate. The spore-bearing surface (hymenium) is smooth. With age, the fruit bodies may become brownish and eventually dry to a thin, varnish-like film. The spore print is white. Microscopic characters The basidia are ellipsoid, longitudinally septate, and measure 10.5–13 × 8–10.5 µm, with a stalk cell up to 30 µm long that becomes enucleate on maturity. The spores are allantoid (sausage-shaped), measuring 8–14 × 3.5–5.5 µm. Hyphae are produced in a gelatinous matrix