PILEUS: 37 - 60 mm wide, "cochin" (MP) (dull fuscous brown) or 6D-F8 in disk and 6C7 or 6E6 toward margin, planoconvex, subumbonate to depressed, slightly viscid when wet; context white, unchanging on exposure, 3.5^± mm thick; margin pectinate-sulcate, decurved; universal veil absent.

LAMELLAE: free, close, white, drying a pale slightly sordid tan or brown (6D-E4), 5^± mm broad, with fimbriate edge that may be white or (at least near pileus margin) marginate (concolorous with pileus surface).

STIPE: 63 - 180 × 6 - 11 mm, entirely white or somewhat darker than 5B3 in upper half and otherwise white, firm, narrowing upward, glabrous to fibrillose; context white, solid or becoming hollow especially in lower portion; exannulate; universal veil as free volval sac, "caramel" (MP) (cinnamon to orange) or whitish, membranous, 20^± × 12^± mm.

Odorless. Taste not recorded.

MACROCHEMICAL TESTS: none recorded.

PILEIPELLIS: 20 - 60 µm thick; slightly gelatinizing subradially arranged interwoven filamentous undifferentiated hyphae with a pale yellowish brown intracellular pigment partially soluble in KOH, 1.2 - 7.8 µm wide; vascular hyphae 1.8 - 11.0 µm wide, branching, relatively common. PILEUS CONTEXT: filamentous undifferentiated hyphae loosely interwoven to tangled, sometimes in fascicles, 1.2 - 12.8 µm wide, with some intercalary segments slightly inflated to 14.0 µm wide; acrophysalides with walls thin or up to 0.5 µm thick, slenderly clavate to subventricose to ventricose (to 92 × 27 µm) to broadly clavate (to 71 × 42 µm) to ellipsoid to subglobose or pyriform (to 63 × 51 µm); vascular hyphae 1.5 - 10.5 µm wide, branching, common. LAMELLA TRAMA: bilateral; central stratum very distinct; subhymenial base with angle of divergence 45° - 90°; filamentous, undifferentiated hyphae 2.1 - 11.5 µm wide, branching; inflated cells up to 34 × 20 µm, clavate to ventricose to ellipsoid, diverging at angles cited above; vascular hyphae 2.2 - 6.3 µm wide. SUBHYMENIUM: 4 to 5 inflated cells deep, with basidia arising from uninflated to slightly inflated hyphal segments or (predominantly) from inflated cells of varying size (up to 27 × 25 µm, clavate to broadly clavate to ovoid to ellipsoid to subpyriform to subglobose to globose, arranged in branching chains arising in central stratum). BASIDIA: 49 - 78 × 11.5 - 24 µm, thin-walled, clavate, dominantly 4-, occasionally 2-sterigmate; sterigmata to 2.0 µm broad at base; clamps rare. UNIVERSAL VEIL: At stipe base, exterior surface: with very thin layer of filamentous, undifferentiated hyphae 2.2 - 12.6 µm wide, in disorderly loose mat, gelatinized, also occasionally in longitudinally oriented fascicles, dominant; inflated cells up to 39 × 33 µm, gelatinizing; vascular hyphae scarce or absent. At stipe base, interior: filamentous, undifferentiated hyphae 1.4 - 15.8 µm wide, rather densely tangled/interwoven, branching, plentiful, locally dominant, at times in fascicles, somewhat gelatinizing; inflated cells more plentiful than at surfaces, locally dominant, subpyriform to ellipsoid to ovoid to broadly clavate to ellipsoid, thin-walled, to 65 × 54 µm; vascular hyphae 2.8 - 7.0 µm wide. On stipe base, inner surface: as rather thin layer, absent in some specimens or in limited areas of volval limb of other specimens; filamentous, undifferentiated hyphae 1.5 - 8.0 µm wide, branching, longitudinally arranged, partially gelatinizing; inflated cells scattered, collapsing, gelatinizing, up to 52 × 34 µm; vascular hyphae 4.2 - 17.0 µm wide, partially gelatinizing, sublongitudinally arranged to disordered. On pileus: absent. STIPE CONTEXT: longitudinally acrophysalidic; filamentous, undifferentiated hyphae 2.0 - 10.5 µm wide; acrophysalides narrowly clavate to narrowly subventricose, up to 290 × 39 µm, with walls thin or up to 0.8 µm thick; vascular hyphae 1.2 - 13.3 µm wide, occasionally tangled in knots.

BASIDIOSPORES: [77/2/2] (10.2-) 11.0 - 14.0 (-15.5) × (9.0-) 9.8 - 13.0 (-13.3) µm (L = 12.5 - 12.8 µm; L' = 12.7 µm; W = 11.4 - 11.9 µm; W' = 11.7 µm; Q = (1.0-) 1.01 - 1.20 (-1.28); Q = 1.08 - 1.09; Q' = 1.09), inamyloid, hyaline, smooth, globose to subglobose to broadly ellipsoid; contents guttulate to granular, in Melzer's reagent coagulating and sticking to spore walls; apiculus sublateral, often broad and prominent, cylindrical; color in deposit unknown.

ECOLOGY: Solitary, at 1700 - 2700 m elev. In Q. humboldtii forests.

Material examined: COLOMBIA: CUNDINAMARCA -- Mpio. Pacho - Los Robledales [misspelled in protologue] above Pacho, 26.vii.1960 R. Singer B3527 (BAFC 31.975, holotype); Cerra de Las Mercedes, rd. btwn. Bogotá and La Mesa, s.d. G. Guzmán 4602 (ENCB & COL, ???paratype???).  NARIÑO: Mpio. Pasto - vereda "La Josefina," km 17, rd. from Pasto to Chachagüí, 20.xi. 1988 A. E. Franco-M. 190 (NY & PSO).

NOTES: Amanita humboldtii is a small brownish mushroom with a striate pileus margin, exannulate stipe, and a membranous whitish to cinnamon colored volval sac. In the present state of knowledge, it is distinguished from A. fuligineodisca Tulloss, Ovrebo & Halling based on microscopic characters.