Summary:
Features include 1) fruiting on hardwood that starts as colorless or white pustules, becoming brain-like or occasionally somewhat leaf-like, spreading out to 10cm or more, becoming vinaceous brown with numerous seed-like concretions embedded in the jelly, 2) spores that are allantoid and colorless, and 3) basidia that are cruciate-septate and myxarioid. Klett(1) says that the presence of spheropedunculate basidia excludes this species from Exidia. The online Species Fungorum, accessed September 9, 2012, and August 30, 2017, gave this as a synonym of Myxarium nucleatum Wallr., Fl. crypt. Germ. (Norimbergae) 2: 260 (1833). However, MycoBank, accessed August 30, 2017, keeps them separate and synonymizes Myxarium nucleatum with Myxarium hyalinum (Pers.) Donk.

Microscopic:
spores 10-11 x 4-4.5 microns, allantoid, colorless, white in mass, germinating by repetition; probasidia 8-12 x 6-8 microns, ovate, becoming cruciate-septate, (Martin), basidia are myxarioid [having a stem-like part separated by a wall from the spherical metabasidial part, as in Myxarium or Stypella]

Notes:
Myxarium atratum is found in BC, MB, NS, ON, PQ, AL, AZ, CA, GA, IA, LA, OH, ME, MI, MN, NC, NJ, NY, VT, WI, and WV, (Ginns), and Panama, Brazil, Europe, Pakistan, and Australia, (Martin).

Synonyms and Alternate Names:
Exidia nucleata (Schwein.) Burt
Naematelia nucleata Peck
Oligoporus leucomallellus (Murrill) Gilb. & Ryvarden
Tremella nucleata Schwein.: Fr.
Tyromyces leucomallellus Murrill