Summary:
Hydropus marginellus is characterized as a Hydropus by the microscopic structure of cap cuticle. It looks generally like a number of brownish Mycenas which have pallid gills that are brown on the edges. The description is derived from Smith(1) except where otherwise indicated. It was described by Smith in Section Hydropus of Mycena, but Hydropus elevated to genus level by Singer.

Cap:
1-2(3)cm across, obtuse to broadly convex when young, with a somewhat incurved margin, sometimes with a rather pronounced umbo, sometimes with somewhat depressed disc; "fuscous" over disc, "drab" or paler toward margin, not fading appreciably; bald, "even or slightly wrinkled, appearing rather dry and velvety", hardly striate but margin frequently cracks or splits radially when old

Flesh:
thin, brittle, watery, if the cap surface is cut, drops of a colorless liquid ooze out

Gills:
broadly adnate to arcuate-subdecurrent, close to crowded (26-35 reach stem), 2-3 tiers of subgills, narrow; pallid with edges sordid brown to fuliginous [soot-colored]; sometimes interveined, edge fringed under a hand lens from projecting cystidia

Stem:
1.5-2.5(3)cm x 0.1-0.2cm, short, equal or the base slightly enlarged, brittle-cartilaginous; ''dark gray to blackish brown at first, becoming grayish brown to almost hyaline gray''; dull and pruinose at first, becoming more or less bald and polished when old; base with scattered mycelial hairs

Microscopic spores:
spores 6-7.5 x 3.5-4 microns, elliptic, smooth, very weakly amyloid; basidia 4-spored; pleurocystidia present only near gill edge and similar to cheilocystidia, cheilocystidia abundant, of two types, saccate and measuring 35-46 x 15-20 microns, or fusoid-ventricose with blunt tops and 40-60 microns, contents of both kinds sordid brownish; cap trama with a covering of irregularly arranged saccate cells measuring 30-40 x 9-20 microns, which have dull brown contents, "their walls occasionally with a thin incrustation when revived in KOH", beneath the surface layer the trama proper composed of very wide hyphae (15-28 microns wide) as well as interwoven, narrow connective hyphae and numerous lactifers 5-12 microns wide, inamyloid; caulocystidia 26-57 x 8-15 microns, subcylindric with obtuse tops or subclavate, filled with dark brown pigment, (Smith), spores 6-7.5 x 3-4.5 microns; basidia 4-spored, 18-27 x 5.5-70 microns, clavate; cheilocystidia abundant, of 2 types, 35-50 x 15-20 microns, saccate to broadly clavate, and 40-60 x 8-12 microns, fusoid-ventricose, obtuse; cap cuticle of saccate to broadly clavate or fusoid-ventricose, thin-walled brown cells measuring 25-50 x 10-20(30) microns (cap cuticle also referred to as subhymeniform with plasmatic pigments); cap trama sarcodimitic, some hyphae inflated, 15-30 microns wide, and some cylindric, 3-8 microns wide, colorless, inamyloid; clamp connections present at base of basidia but absent or rare in tramal tissues, (Castellano), spores 5.7-7.7 x 3.1-4.7 microns, elliptic, smooth; basidia (2-)4-spored, 18-27 x 5.5-7 microns, clavate, with basal clamp connection; cheilocystidia cylindric to ventricose, 30-50 x 7-11 microns; cap cuticle of irregular hyphae, with erect, cylindric to clavate or fusiform ends 3-17 microns wide (dermatocystidioid), septate, no clamp connections observed, (Breitenbach)

Spore deposit:
white (Castellano)

Notes:
Hydropus marginellus has been found at least in WA, NC, and Europe, (Smith). It has also been identified from OR and CA, (Castellano). There are collections from BC at the University of British Columbia.

EDIBILITY

Synonyms and Alternate Names:
Mycena marginella (Fr.) Quel.