Summary:
{See also Stereum Table.} Features include 1) fan-shaped, semicircular, or irregular caps on mainly hardwoods (may be bent outward from resupinate surface), 2) upper surface of cap that is tomentose-hairy, zoned and multicolored, light buff to rusty brown or grayish, with a tomentose-felty surface, 3) spore-bearing surface that is buff to light brown or grayish, smooth, often undulate, when damaged bleeding or not (the fluid may be red or yellow turning red), 4) spores that are narrowly elliptic to cylindric, smooth, and colorless, 5) pseudoacanthohyphidia, thick-walled cystidia (and sometimes conducting hyphae), and presumably sharp-tipped hyphidia, and 6) hyphae of two types, by some authors referred to as generative and skeletal. |Stereum subtomentosum Pouzar may be the correct name for many collections labeled Stereum ostrea from the Pacific Northwest: Ginns(5) says of S. subtomentosum "We suspect that this is a common species and specimens have been misnamed S. ostrea" (Latin name in italics). |Stereum ostrea, Stereum rugosum, and Stereum sanguinolentum are members of the subgenus Aculeatostereum which have pseudoacanthohyphidia: thin-walled hymenial elements that bear a few (2-5, rarely up to 10) apical projections (also known as pseudoacanthophyses, aculeate-tipped basidioles, acanthocystidia, and acanthohyphidia, by different authors, the last term also used for acanthophyses - with projections throughout their length - that occur in subgenus Acanthostereum not found in North America), (Chamuris(3)). |Stereum ostrea is common in North America south of latitude 42 degrees north, (Chamuris).

Microscopic:
SPORES 4-7(8) x 2-3(3.5) microns, narrowly elliptic to cylindric; thick-walled CYSTIDIA always present, conducting hyphae sometimes present; PSEUDOACANTHOHYPHIDIA thin-walled; generative hyphae 2-3 microns wide, skeletal hyphae 4-6 microns wide, (Julich), SPORES 5.5-7.5 x 2-3 microns, cylindric, smooth, spore print white, (Phillips), SPORES 5.5-7.5 x 2-3 microns, cylindric to somewhat flattened on one side, smooth, colorless, spore print white, (Lincoff), spores 4-7 x 2-3 microns; PSEUDOACANTHOHYPHIDIA present and [on the basis of subgenus definition] acuminate-tipped (sharp-tipped) HYPHIDIA, (Chamuris)

Notes:
Stereum ostrea has been found in BC, WA, OR, ID, MB, NB, NF, NS, ON, PE, PQ, AL, AZ, CA, CO, CT, DC, FL, GA, IA, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MS, MT, NC, ND, NM, NJ, NY, OH, OK, PA, SC, SD, TN, TX, VA, VT, and WI, (Ginns).