Domain: Eukarya
Kingdom: Fungi
Phylum: Basidiomycota
Class: Agaricomycetes
Order: Agaricales
Family: Agaricaceae
Genus: Agaricus

Original English description:
“Pileus 5–8(–10) cm broad, at first broadly convex, with moderately inrolled margin, becoming plane, disk sometimes slightly depressed; surface dry, glabrous on disk, elsewhere innately fibrillose, color pallid to sordid (when young) or medium brown, if brown then becoming appressed fibrillose-squamose, squamae ca. 10 mm long 3 5–15 mm broad, background color whitish to pale buff; context white, becoming reddish near lamellae and stipe apex when sectioned, firm, up to 10–16 mm thick, odor of A. bisporus plus fruity/spicy as in some species of Sanguinolenti after exposure. Lamellae free, close, up to 10 mm broad, at first pallid to dingy pinkish, finally dark blackish-brown, margin somewhat pallid. Stipe 3–8 cm long 3 1–2 cm broad, equal to subclavate, occasionally ventricose (in culture), interior white, quickly becoming orange-red, ultimately red after longitudinal sectioning, in median transverse section becoming orangish, stuffed-hollow; surface glabrous, or minutely floccose or with fibrillose zones below, white, coloring as above when incised; base firmly rooted in litter and/or soil. Veils forming a supramedian, white intermediate-type annulus with a pendant aspect, sheathing upward and with a broadly flaring limb, unrolling with the pileus margin, upper surface smooth to striate, lower surface interwoven to scurfy, sometimes subtended by one or two small limbs, margin thin, uneven; universal veil also sometimes leaving a scale ring above the base of the stipe. Spores (4.5–5.3–)5.7–6.5(–7.1–7.5) 3 (4.1–)4.4–4.7(–5.3–6.0) mm, dark brown, broadly ellipsoid to ellipsoid, hilar appendix semiprominent, no apical pore evident. Basidia 17–28.5 3 (4–)7–9(–12) mm, clavate to cylindro-clavate, tetrasporic; sterigmata 1–4 mm long. Cheilocystidia 15–33(–45) 3 3–15 mm, clustered, not abundant; lamellar margin primarily composed of hyphae oriented parallel to the margin, sterile.”
Mycologia, 100(6), 2008, pp. 876–892.

An excellent edible. Tastes similar to, but better than a commercial ‘portobello’ (mature, commercially grown A. bisporus).

http://www.indexfungorum.org/...

Orignal latin diagnosis:

“Pileus 5–8(–10) cm latus, late convexus, mediocriter fuscus vel raro subalbidus; pileipellis implexa, levis vel paulum squamosa, continuo integra in margine, cetera autem parte sordide aurantiaca evadente; stipes subaequalis, 3–8 cm longus 3 1–2 cm latus, externa parte levi, cetera autem parte paulum fibrata atque medullosa in medio, colorem sordide aurantiacum ducit; sporae circa 5.7–6.5 3 4.4–4.7 mm, late ellipsoideae vel tantum ellipsoideae, basidia tetrasporigera, cheilocystidia cylindracea forte etiam claviformia, 15–33 3 3–15 mm. Dispersus sub coniferis crescit in litoris regionibus parva altitudine. HOLOTYPUS: Point Lobos State Reserve, Monterey County, California, USA: RWK 1397, 10 Jan 1986, in herbario SFSU depositur. MB 512235.”
Mycologia, 100(6), 2008, pp. 876–892.

Agaricus agrinferus Kerrigan & Callac, Mycologia 100(6): 886 (2008)