Summary:
Inocephalus rhombisporus is identified by a dark brown, translucent-striate cap with a scum-like - felty disc, a pale brown, fragile stem darkening to medium or dark brown, the stem pruinose to finely scaly near the top, mostly 4-sided angular spores, pleurocystidia and cheilocystidia. The description is derived from Largent(1) who says it is common.

Cap:
1.3-3cm across, conic or parabolic to flat, rarely slightly depressed, often umbonate, margin incurved to decurved [downcurved] to flat; hygrophanous, dark brown when young and remaining so on disc, elsewhere becoming medium brown or lighter (brownish orange); typically disc scum-like [having superficial dark-colored fibrils] to felty-tomentose, elsewhere bald (faintly appressed-fibrillose under a hand lens), translucent striate halfway or most often all the way to disc, margin "entire, then eroded and eventually minutely fringed"

Flesh:
up to 0.2cm thick near gill attachment; colored as cap surface

Gills:
adnexed, subdistant, narrow to moderately broad, 0.2-0.6cm broad; white at first [presumably becoming pinkish from spores], edges most often lighter than faces; edges smooth

Stem:
5.5-13cm x 0.15-0.35cm at top, 0.15-0.4cm wide at base, equal to rarely widened downward, round in cross-section or often flattened, very fragile, hollow; very pale brown to pale tan to whitish, often darkening to medium brown from base up; pruinose to scabrulose [finely scaly] at top and often upper half, bald lower half, basal mycelium absent to scarce

Veil:
[absent]

Odor:
"indistinct to fungoid, grassy, somewhat spermatic, or even very slightly farinaceous, however never strong"

Taste:
indistinct to faintly farinaceous

Microscopic spores:
spores 6.7-11.8 x 5.6-10.5 microns, mostly 4-sided, rarely 5-6 sided, more or less square to rectangular, smooth; basidia 4-spored, 33.8-64.2 x 8.4-13.5 microns, easily separated when squashed; pleurocystidia rare to scattered or often absent in a few sections, 54.7-74.3 x 8.0-17.8 microns, cheilocystidia "abundant and projecting beyond hymenium", 31.3-77.7 x 4.9-18.5 microns, "fusoid-ventricose to rostrate-ventricose, colorless"; cap cuticle "a layer of loosely entangled hyphae to a loosely entangled trichodermium on disc, with age forming agglutinated clumps, elsewhere a cutis", pileocystidia 26.5-97.7 x 4.5-10.6 microns, cylindro-clavate; clamp connections present at base of basidioles and basidia but very difficult to see, rare in cap cuticle; pigmentation intracellular and dominant as well as pale brown and uniform in cap cuticle, hyphae of cap cuticle "often at times with what appears to be membranal pigmentation", "at times minutely and internally or externally incrusted"

Spore deposit:
[presumably pinkish brown]

Notes:
Inocephalus rhombisporus has been found at least in WA, OR, ID, and CA, (Largent). It has been reported by Oluna Ceska from BC (collection at the University of British Columbia).

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Synonyms and Alternate Names:
Leptonia rhombispora Kuehner & Boursier