Summary:
Lentinellus ursinus, when considered in a wide sense (see below) is recognized by a shell shape with little or no stem, a brown cap color with a velvety to fuzzy surface at least in center, gills with ragged or coarsely toothed edges, a peppery taste, growth on wood, a white spore deposit, and small, nearly round, amyloid, spiny spores. There are competing concepts of Lentinellus ursinus: Miller(10) in 1971 included as 2 synonyms Lentinus castoreus (Fr.) Romagn. and Lentinellus pusio Romagn., but Petersen(25) in 2004, aided by molecular and sexual compatibility work, keep the 2 synonyms as Lentinellus castoreus, whereas Watling(2) says in 1989 "the closely related L. castoreus (Fr.) Konr. & Maubl. differs in its paler rather smaller fruit-bodies and preference for conifer wood. Some authorities consider them the same.", and Arora(1) does not mention L. castoreus. The differences between the Petersen(25) concepts of L. ursinus and L. castoreus are given under SIMILAR. The descriptions below represent the concepts of each set of authors.

Cap:
6-8.4cm broad, 6.5-8cm long, "hoof-shaped or irregularly lobed shell-shaped", often depressed either in center or around point of attachment; "umber, bay or date-brown with paler cinnamon margin, drying entirely fulvous-buff"; entirely velvety tomentose except for +/- smooth incurved margin, "tomentum pale buff when dry but rapidly soaking up water" to become +/- concolorous, slightly radially rugulose [wrinkled] towards margin, (Watling), (1.5)3-10cm along substrate, projecting 2-5cm, convex to nearly flat, margin incurved at first to nearly straight when old; alutaceous to "cinnamon" over margin, elsewhere dark brown, "russet", "fawn color" to "sayal brown", "the woolly pubescence usually lighter and often a mustard brown but in old age near the point of attachment the hispid hairs become blackish brown"; surface variable: nearly bald to woolly over margin, velvety-tomentose or even hispid (bristly) over point of attachment, (Miller(10)), 3-10cm along substrate, projecting 2-5cm, kidney-shaped to fan-shaped, convex becoming flat, margin incurved at first and often lobed; dark brown to brown, yellow-brown, or reddish-brown, often paler on margin; with sparse to dense brown to dark brown fine hairs, at least toward the stem, dry, margin usually smooth, (Arora), up to 10cm x 6cm, "laterally attached, pleurotoid, crepidotoid, to more or less conchate", "margin inrolled at all ages, entire or lobed, often ?hygrophanous within 0.5 mm and/or staining deep brown, usually appendiculate by 1-2 mm in youth"; cap surface "minutely strigose, velutinous, pilose, pruinose, tomentose to densely hirsute (near attachment)" over at least inner 1/3 of cap surface (when imbricate, cap surface of lower caps may be smooth), and in this part of cap "tawny olive", "sayal brown", "bister", "warm sepia", "snuff brown", "Natal brown", brown-black, "avellaneous" to "wood brown", outward smooth to delicately radially rivulose [with river-like lines] or suede-like, and in this area of the cap '"tilleul buff," "avellaneous," "cinnamon buff," "wood brown," "pinkish buff," to "tilleul buff," often with small dark brown spots or blotches', (Petersen), dark brown to cinnamon (Miller(14))

Flesh:
thin, except at attachment; "pale date brown or horn-colored under cap cortex", pale cream to grayish cream-buff below, horn-colored over gills, (Watling), 0.1-0.3cm thick, firm, fleshy, a dense woolly brown cap cuticle creates a duplex effect when sections are made through non-bald areas of cap; white, "pinkish buff" to "avellaneous", (Miller(10)), thin, (Arora), flesh pale buff to sand color; ligne noire [black line representing cap cuticle between suprapellis and cap flesh] often, but not always present, varying in depth and color (Petersen)

Gills:
radiating from point of attachment, subcrowded; "whitish, pale cream or creamy-buff, often becoming darker cinnamon buff or umber at point of attachment, pale milky coffee or pale fulvous buff when old"; edge slightly to distinctly dentate, (Watling), radiating from point of attachment, often decurrent when base is broad, close to subdistant, broad; light pink, "avellaneous", pale vinaceous buff to darker dingy "vinaceous-buff" to "pale pinkish cinnamon", light reddish brown to light brown when dry; serrulate to coarsely serrate [finely to coarsely saw-toothed], sometimes lacerate when old, (Miller(10)),decurrent (if stem present), close, broad; dingy white to pinkish brown; with ragged or coarsely toothed edges, (Arora), "adnate to adnexed, subdistant to close but not crowded", up to 0.5cm broad; when young "light ochraceous buff" to "pale pinkish cinnamon", with grayish olive hues, mellowing to "cinnamon buff", "ochraceous buff", "pinkish buff", "tilleul buff", "pale cinnamon pink", "clay color", sometimes with small orange-brown stains; edges "almost entire to weakly undulate-serrate, to finely serrate-digitate throughout length", (Petersen)

Stem:
"rudimentary or replaced by broad, irregularly shaped woody base" 0.8-2cm broad, (Watling), sessile (without a stem), (Miller(10)), absent or rudimentary (Arora), without discernible stem, attachment broad, represented by gray-brown tomentose attachment, (Petersen)

Veil:
absent (Arora)

Odor:
"fairly strong, acid-fragrant, peculiar", (Watling), usually fruity, sometimes fungoid, (Miller(10)), weakly to mildly aromatic, somewhat peppery, (Petersen), not distinctive (Phillips)

Taste:
peppery, sometimes developing slowly after chewing, (Watling), strongly peppery, sometimes slowly but most often instantly, (Miller(10)), strongly bitter (Miller(14)), slowly peppery or bitter (Arora), immediately to quickly strongly acrid [peppery], (Petersen), strongly peppery (Phillips)

Microscopic spores:
spores 3-4 x 2.5-3 microns, nearly round or broadly oval, minutely asperulate [roughened with tiny points] or appearing smooth, amyloid; basidia 4-spored, 12-25 x 3-7.5 microns, narrowly clavate, colorless in water and alkali; pleurocystidia 16-30 x 3.5-5 microns, either fusiform or simply narrowly clavate, colorless or slightly honey-colored in water, cheilocystidia 15-28 x 3-5 microns, "cylindric-flexuous but often slightly swollen towards base", lageniform or +/- fusiform, often flexuous [wavy] with apex 1-2.5 microns broad; clamp connections present, (Watling), |spores 3.0-4.6 x 2.0-3.5 microns, nearly round to very short-elliptic, with minute amyloid echinulations [small spines], colorless to light yellowish in KOH, thin-walled; basidia 4-spored, 12-25 x 3.0-7.5 microns, narrowly clavate, colorless in KOH or Melzer''s reagent; pleurocystidia 16-30(45) x 3.5-5.0(7.5) microns of 2 types: 1) protruding somewhat, fusiform, thin-walled, colorless in KOH or Melzer''s, 2), usually protruding somewhat, narrowly clavate, obtuse at apex, "occasionally with irregular dense yellow areas in Melzer''s giving the appearance of thick-walled cells, pale yellow to hyaline with the appearance of internal folding in KOH", cheilocystidia 16-25 x 2-5 microns, "similar in every way to the pleurocystidia but usually less numerous, often hard to find", (Miller(10)), |spores 2.5-5 x 2-3.5 microns, nearly round, with minute amyloid spines, (Arora), |spores 4-4.5 x 3.0-3.5 microns, broadly elliptic to nearly round, strongly amyloid, ornamentation of densely scattered verruculae visible at 1500x; basidia [shown as 4-spored], 18-22 x 5.3-5.8 microns, emergent up to 8 microns, narrowly clavate, with clamp connection, hymenium composed of 2 other elements: 1) pleurocystidia rare, from hardly emergent to emergent up to 8 microns, 16.5-20 x 5-6 microns, fusiform to spheropedunculate with lanceolate apex, colorless, thin-walled, 2) gloeocystidia, emergent up to 8 microns, 20-32 x 6.0-6.8 microns, clavate to clavate-digitate, geniculate, dull golden refringent in KOH; cheilocystidia "as a fringe of slender hyphal tips, vermiform, spheropedunculate with lanceolate apex, gnarled, digitate", emergent from gill edge up to 10 microns, colorless, thin-walled, with clamp connections; cap cuticle "a repent layer of generative hyphae apparently without interhyphal mucus", hyphae 2.6-5.6 microns wide, colorless, thin-walled to thick-walled (wall up to 0.7 microns thick), "frequently clamped, occasionally with minute spiculate ornamentation which can appear delicately striate", outermost layer (suprapellis) a thatch of slender hyphae toward cap margin, inward producing fascicles (emergent up to 500 microns) of erect filamentous hyphae, hyphae 2.1-2.6 microns wide, almost colorless singly, brown in mass in KOH, "in fascicles of more than 100, frequently clamped, inamyloid"; cap trama trimitic 1) generative hyphae 3.5-6.5(9.5) microns, colorless, thin-walled to thick-walled, wall up to 0.7 microns thick, inamyloid, frequently clamped, 2) skeletal hyphae 4.5-7.5 microns wide, glassy-refringent in KOH, gently flexuous [wavy] to bosselees, [undulating to zig-zag with amyloid knobs at angles], weakly, moderately to strongly amyloid, unbranched or rarely with a spur-like branch, aseptate, and 3) gloeoplerous hyphae 3.5-5.5 microns wide, hardly refringent in KOH, pallid yellowish, thin-walled, contents resinous, (Petersen)

Spore deposit:
white (Watling, Arora, Miller(10))

Notes:
Miller(10) examined collections (in the wider sense) from WA, ID, QC, AL, AZ, CA, DC, FL, KY, LA, MA, MD, ME, MI, MS, NH, NJ, NM, NY, OH, PA, TN, TX, VT, Austria, France, Sweden, and India. Petersen(25) examined collections (in the narrow sense) from WA, ID, AZ, CA, GA, IA, IL, IN, KY, MD, MI, MN, MO, NC, NJ, NY, OH, PA, SC, TN, WI, WV, Costa Rica, Mexico, Colombia, Austria, Finland, France, Poland, Russia, and Sweden. There are collections from BC at Pacific Forestry Centre and University of British Columbia, but it is unknown whether these fit the Petersen(25) concept of L. ursinus or their concept of L. castoreus.

EDIBILITY

inedible due to bitter or peppery taste (Arora)