Summary:
Heterogastridium pycnidioideum is minute and unlikely to be encountered, but of intrinsic interest in its own order, with basidiospores that have 4 arms released from the interior of a flask-shaped apparatus, described in 1990.

Microscopic:
spores tetraradiate or rarely pentaradiate, (with 4 or 5 arms), the whole spore measuring 15-25 microns in outline and the arms 2-3(4) microns in diameter, conidia are also mixed with the basidiospores; basidia long-cylindric, 60-80 x 5-8 microns, transversely 3-septate when mature, each basidium developing 1 basidiospore; hyphae 2-3(5) microns wide, colorless; the single layer of hyphae forming the base and neck of the flask-shaped fruiting body (pycnidium) consists of hyphae 8-15(20) microns in diameter and 100-500(800) microns long

Notes:
It is found in its anamorph form (Hyalopycnis blepharistoma) from North America (at least BC, NY, LA), Austria, United Kingdom, and Japan.