Summary:
Principal fieldmarks are 1) fragile texture, 2) a smooth hygrophanous, usually convex cap that is dark red brown to orange brown, drying tan, 3) a white or pallid stem that lacks a ring, 4) growth in large clumps on decaying hardwood, 5) small, dark brown spores, 6) rounded to obtuse cystidia, and 7) cap flesh hyphae that are dark cocoa in KOH. The Smith(5) description used here is for P. hydrophila (Fr.) Maire which is given as a synonym of P. piluliformis (Bull: Fr.) P.D. Orton by Breitenbach(4), by Hansen, L.(2), and by the online Species Fungorum, accessed April 13, 2015. Psathyrella piluliformis is common.

Cap:
2-5(7)cm across, obtusely conical or convex when young, becoming broadly convex, finally nearly flat, occasionally obscurely umbonate; hygrophanous, reddish brown or occasionally somewhat umber as spores mature, and when old at times near blackish brown when wet, pale tawny over marginal area, fading to various shades of pale tan, rarely vinaceous-brown fading to pale cinnamon; moist, bald or white-fibrillose only along margin, the margin at first fringed or with a belt of fibrils from veil, sometimes translucent-striate when moist, occasionally decidedly rugulose [finely wrinkled], (Smith), 2-5(7)cm across, "bluntly conical or more often convex when young", expanding to nearly flat when old; hygrophanous, dark reddish brown to orange-brown or rusty brown when moist, "fading as it dries to pale tan or sometimes grayish", margin often darker brown; margin often hung with veil remnants, (Arora), "dark red-brown when moist and yellow-brown to ocher-brown when dry", (Trudell)

Flesh:
rather firm and rigid, becoming fragile; "watery brown fading to pallid tan", (Smith), thin, fragile, (Arora)

Gills:
crowded, narrow to moderately broad, equal; pale brownish becoming dark reddish brown at margins; margin even or only slightly fringed, (Smith), adnate to adnexed, crowded, buff to pale brown, becoming chocolate brown or dark brown when old, (Arora)

Stem:
3-7(15)cm x (0.2)0.3-0.6(0.8)cm, equal, hollow, not distinctly fragile except for small fruiting bodies; white to pale grayish at first, sordid brownish when old at least near base; top pruinose, lower part somewhat fibrillose from veil remnants but becoming bald, (Smith), 3-7(10)cm x 0.2-0.6(1)cm, equal, fragile, hollow; white to faintly grayish or sometimes brownish when old, especially in lower part; smooth, (Arora)

Veil:
fibrillose to submembranous, (Smith), fibrillose, fleeting, leaving remnants on cap margin and occasionally an obscure zone of hairs on stem, (Arora)

Odor:
not distinctive (Smith)

Taste:
not distinctive (Smith), some collections are bitter (Arora)

Microscopic spores:
spores (4)4.5-5.5(6) x 3-3.5 microns (or 5.5-7 x 3.2-3.7 microns from deposits), elliptic to slightly ovate in face view, subelliptic to slightly bean-shaped in side view, smooth, apical pore present but inconspicuous, color in KOH pale to dark cocoa slowly becoming chocolate-gray, in Melzer''s reagent pale tawny slowly becoming reddish tawny, wall about 0.2 microns thick, (Smith); basidia 4-spored, 14-16(20) x 5.5-7 microns, clavate, colorless in KOH; pleurocystidia abundant, (30)36-48(56) x 9-16(20) microns, "broadly fusoid-ventricose with the neck more or less elongated and ending in a broadly rounded to obtuse apex, thin-walled, smooth", colorless, content not distinctive, cheilocystidia 22-30 x 9-14 microns, clavate to saccate, colorless and thin-walled, cells similar to pleurocystidia also present; clamp connections present, (Smith), spores 4-6 x 3-4 microns, elliptic, smooth, (Arora), spores 4.5-6 x 3-4 microns, cystidia rounded to obtuse, "hyphae of the cap flesh that are dark cocoa-colored" when mounted in KOH, (Trudell)

Spore deposit:
deep brown (Arora), dark purple brown (Breitenbach)

Notes:
Collections were examined from BC, WA, OR, NS, ON, QC, CA, MD, ME, MI, MS, NC, NJ, NY, OH, PA, and TN, and distribution given as "Throughout the United States and southern Canada", (Smith). It also occurs in Europe, including France and Switzerland. Breitenbach(4) give the distribution as distribution North America, Europe, Asia, and North Africa.

EDIBILITY

not recommended (Arora)

Synonyms and Alternate Names:
Psathyrella hydrophila (Bull.) Maire