E-Flora BC: Electronic Atlas of the Flora of British Columbia

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.

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).

Fruiting body:
fruiting starts as colorless or whitish pustules, at first erumpent [bursting through surface of substrate], cerebriform [brain-like], or occasionally subfoliate [somewhat leaf-like], soon anastomosing and becoming broadly effused [spread out], up to 10cm or more in greatest extent; "becoming vinaceous brown and with numerous seed-like calcareous concretions" 0.02-0.05cm in diameter embedded in the jelly, "drying to a thin dark film with the concretions very prominent"; it is also stated that color changes from pure white through vinaceous to brown, (Martin), very occasionally with a pink tinge; spore deposit white (Buczacki)

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]