er 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.

Sporangia scattered, dull grayish-yellow or gray, depressed-globose,
umbilicate, minute, stipitate; the peridium comparatively thick,
tenacious, especially persistent below, tawny or yellow; the stipe pale
brown or orange, erect, even or slightly enlarged at base; hypothallus
scant or none; columella prominent, more or less discoidal, rough, or
spinulose, especially on the upper surface, yellow; capillitium not
abundant, pale fuliginous, often branching and anastomosing so as to
form a loose net; spores nearly smooth, dark violaceous by transmitted
light, 8.5-9.5 mu.

The species differs from _D. xanthopus_ in several particulars,