Mucilopilus castaneiceps (Hongo) Har. Takah., Trans. Mycol. Soc. Japan 29(2): 119 (1988)         
　Basionym: Tylopilus castaneiceps Hongo, J. Jap. Bot. 60: 375. 1985.
      Pileus 2-8 cm broad, hemispherical to convex at first, becoming plano-convex to applanate at maturity; surface viscid when wet, glabrous, often partly rugose-scrobiculate with age, when young evenly colored brown (7D6-7D7, or 7E6-7E8), then somewhat paler toward the slightly projecting margin with age. Veil absent. Tubes 5-10 mm deep, adnate to subdecurrent, at first whitish,　pink flesh to dark pink at maturity, unchanging when cut; pores small, 1-3 per mm, subangular, concolorous with the tubes, unchanging when bruised, often exudes or is beaded with droplets of a hyaline liquid when young and fresh. Stipe 5-10 cm long, 0.7-1.3 cm thick, slender, subequal or somewhat thickened toward both ends or tapering apically, often irregularly curved, solid; surface viscid when wet, glabrous, whitish, often with yellow (3A7) to vivid yellow (3A8) stains here and there when mature, slightly or inconspicuously reticulate above or only at the apical portion but in rare cases finely reticulate overall, often longitudinally striated or ridged downward. Flesh at first firm, later soft with age, 5-15 mm thick at the disc of the pileus, whitish with a pale brownish tint beneath the pileal cuticle, at maturity with yellow (3A7) to vivid yellow (3A8) stains especially in the stipe; odor none, taste bitter.
　   Basidiospores in deposit brown (6D7-6D8, 6E7-6E8), 10-13(-15)×4-5 µmＥm = 2.5 (median length/width ratio), ellipsoid to subcylindric, inequilateral with a suprahilar depression in side view, smooth, melleous in KOH, yellow tan to reddish brown in Melzer's. Basidia 20-35×5-7.5µm, clavate to broadly clavate, 2-4 spored, hyaline in KOH, pale brownish yellow in Melzer's. Cheilocystidia abundant on tube edges, 40-80(-100)×6.5-10µm, cylindrical with a rounded apex, often 1-2 septate, thin-walled, hyaline in KOH, pale brownish yellow in Melzer's. Pleurocystidia rarely present, 25-40×7.5-12µm，fusoid-ventricose to subclavate, thin-walled, hyaline in KOH, pale brownish yellow in Melzer's. Tube trama bilateral of the Boletus-type. Pileus cuticle composed of an ixotrichodermium of　loosely　interwoven　hyphae;　end cells 3-6µm broad, cylindrical with coagulated contents, hyaline to pale melleous in KOH, pale brownish yellow in Melzer's. Stipe surface consisting of continuous layer of caulocystidia 50-80×6-10µm, cylindrical, often 1-2 septate, thin-walled, hyaline in KOH, pale brownish yellow in Melzer's, with brownish incrustations (in KOH). Clamp connections absent.
      Specimens examined: On ground in Quercus serrata - Quercus acutissima forests, Mt. Jinba, Tsukui-gun, Kanagawa, 20 June 1982, H. Takahashi, TUAT 153; 17 July 1983, H. Takahashi, TUAT 193; 16 Oct. 1983, H. Takahashi, TUAT 211; 16 July 1985, H. Takahashi, TUAT 440; on ground under Quercus serrata, Quercus glauca, and Quercus myrsinaefolia, Mt.Takao, Hachioji, Tokyo, 30 June 1985, H. Matsui & H. Takahashi, TUAT 370; 24 Aug. 1985, H. Matsui & H. Takahashi, TUAT 524; 24 July 1986, H. Takahashi, TUAT 676; 16 Aug. 1987, H. Takahashi, TUAT 732.
      Distribution: Japan (Tokyo, Kanagawa, Kyoto).
      The specimens cited above agree well with the Hongo's original description (1985) of this species. According to Hongo (1985), it is said to occur in Pinus densiflora and Quercus serrata forests on the outskirts of the city of Kyoto, Kyoto-Prefecture. In eastern Japan (Tokyo, Kanagawa), it has been collected from oak forests, especially under Quercus serrata, Quercus acutissima, Quercus glauca, and Quercus myrsinaefolia, from late June to October.
      The pileal ixotrichodermium, the brown spore print and the pleurocystidia with contents macrochemically non-reactive to KOH and Melzer's reagent are all characteristic of the genus Mucilopilus Wolfe (1979),　although the basidiospore Ｅm value is much smaller than that originally given by Wolfe for the genus as 3.0-3.5.　Hongo (1985) preferred to accommodate this species in　Tylopilus rather than Mucilopilus because of the smaller basidiospore Ｅm value. However, Wolfe (1982) has already recognized a taxon which has the same basidiospore Ｅm value as in M. castaneiceps, Mucilopilus mexicanus (Guzmán) Wolfe in this genus. Therefore, the difference only in the basidiospore 　Ｅm value does not prevent to put this fungus into Mucilopilus.　M. mexicanus has a similar basidiospore measurement but apparently differs in its whitish basidiocarp with a non-reticulate stipe and in its lignicolous habitat (Guzmán, 1974).

Literature cited
Guzmán, G. 1974.　El genero Henn. (= Ixechinus) y las  relaciones floristicas Mexico y Africa. Bol. Soc. Mex Mycol. 8: 53-63.　

Hongo, T. 1985.　Notes on Japanese fungi (23). J. Jap. Bot. 60: 370-378.

Kornerup, A. and Wanscher, J. H. 1978.“Methuen handbook of colour," 3rd. ed., & Co., London. 252 p.

Wolfe, C. B. 1979.　Mucilopilus, a new of the Boletaceae, with on North American taxa. Mycotaxon 10: 116-132.

Wolfe, C. B. 1982.　A taxonomic evaluation of the generic status of Ixechinus and Mucilopilus (Ixechineae, Boletaceae). Mycologia 74: 36-43.

(Mycoscience 40: 73-80, 1999)