Summary:
Postia ptychogaster may be found in its non-sexual stage (with chlamydospores) which typically precedes formation of the tube layer and pore surface (with basidiospores). The non-sexual (imperfect) stage is a soft, cream-colored cushion covered with minute spines, and its core becomes powdery gray-brown. The sexual fruitbody, up to several centimeters in diameter, often develops on the underside of this cushion and can form a semicircular or spatula-shaped cap or a spread-out layer on the substrate. There is sometimes a lateral stem. The cap when present is white and downy, while the pore surface is pallid. The description is derived from Ginns(28).

Microscopic:
basidiospores 3.8-4.4 x 2.4-2.8 microns, elliptic to nearly oblong, "some narrowed slightly over the apical one-third", smooth, inamyloid, colorless, walls about 0.3 microns thick, cyanophilic, apiculus minute; chlamydospores 5.6-8.0(10.0) x 3.5-6.0 microns, elliptic to nearly oblong, "some with ends truncated", smooth, dextrinoid, pale yellow brown, cyanophilic, walls about 0.4 microns thick; hyphae with clamp connections, walls colorless and thin, of two types: 1) "in the cottony context typically curved", 2-3 microns wide, with "prominent clamp connections", and 2) "in the dense context adjacent to the tubes and in the strands, essentially parallel, straight", (2.4)3.0-5.0(8.0) microns wide; tramal hyphae "straight, vertically woven", 3-4 microns wide, walls typically 0.5 microns thick, "en masse weakly to moderately intensely dextrinoid"

Notes:
Postia ptychogaster is known in western North America from BC, WA, OR, and CA.

Synonyms and Alternate Names:
Athelia septentrionalis J. Erikss.
Fibulomyces septentrionalis (J. Erikss.) Julich Beih.