Sporangia gregarious, globose or hemispheric, umbilicate beneath, small, white, stipitate; the peridium smoky, covered with minute calcareous crystals; stipe slender, erect, black, opaque; hypothallus scutate, black; columella distinct, globose, black or dark brown; capillitium of delicate threads, pale brown or colorless, with occasional brown thickenings or nodes, sparingly branched; spores pale, violaceous by transmitted light, minutely warted, 6-8 mu. This is _D. microcarpon_ Rost. Fries, _l. c._, acknowledges the priority of Link's appellation, and discards _microcarpon_. Rostafinski adopted _microcarpon_ simply because he thought it more appropriate. Fries describes the columella "none or black." It is doubtful whether we have the typical Friesian form on this continent. The fructification is in our specimens small, about .4 mm., and the spores, as noted by Morgan, small; otherwise the species is hardly more than a variety of the next. Under the name _D. nigripes_ Lister groups our Nos. 10, 11, 12. _N. A. F._, 1393, represents Dr. Rex's conception of the present species. Not common. New York, Ohio, Iowa. 11. DIDYMIUM XANTHOPUS (_Ditmar_) _Fr._ PLATE XVI., Fig. 10. 1817. _Cionium xanthopus_ Ditmar, Sturm, _Deutsch. Fl._, III., p. 37, t. 43. 1829. _Didymium xanthopus_ (Dit.) Fr., _Syst. Myc._, III., p. 120. 1873. _Didymium proximum_ Berk. & C., _Grev._, II., p. 52. 1892. _Didymium microcarpon_ (Fr.) Rost., Macbr., _Bull. Lab. Nat. Hist. Iowa_, II., p. 146, in part. 1894. _Didymium nigripes_ Fr., List., _Mycetozoa_, p. 98, in part. Sporangia gregarious, white, globose, slightly umbilicate, stipitate; the peridium thin, and nearly or quite colorless, frosted with crystals of lime; the stipe yellowish or yellowish brown, corneous, erect, subulate, slender; hypothallus none; columella pale or white, turbinate, globose or depressed-globose; capillitium of dull brown, or colorless threads more or less branched, always white at the tips; spores violaceous, nearly smooth, 7.5-8.5 mu. This seems to be the most common form in the United States. It is distinguished from the preceding by the longer, more delicate, generally orange-yellow, stem with pale or white columella. The spores also average a shade larger. _N. A. F._, 412 and 2089, are illustrations of _D. xanthopus_. The columella in blown-out specimens is very striking, well confirming the diagnosis of Fries, "_valde prominens, globosa, stipitata, alba_." Berkeley makes the color of the capillitium diagnostic of _D. proximum_, but this feature is insufficient. Eastern United States; common. 12. DIDYMIUM EXIMIUM _Peck._ PLATE XVI., Figs. 11, 11 _a_, 11 _b_. 1879. _Didymium eximium_ Peck, _Rep. N. Y. Mus._, XXXI., p. 41.