p. 114.
1844. _Didymium clavus_ (Alb. & Schw.) Rabh., _Ger. Cr. Fl._,
No. 2282.
1875. _Didymium clavus_ (Alb. & Schw.) Rost., _Mon._, p. 153.
1899. _Didymium clavus_ (Alb. & Schw.) Rabenh., Macbr., _N. A. S._,
p. 90.
1911. _Didymium clavus_ Rost., List., _Mycet., 2nd ed._, p. 128.

Sporangia gregarious, pale gray, discoid or pileate, depressed,
stipitate; the peridium dark-colored, frosted with calcareous crystals
above, naked below; stipe short, slender, tapering upward, furrowed,
arising from a hypothallus more or less distinct, black; columella
obsolete; capillitium of delicate threads, pale or colorless, little
branched; spores violaceous, pale, nearly smooth, 6-8 mu.

This species is well differentiated, easy of recognition by reason of
its peculiar discoid sporangium, calcareous above, naked and black
beneath. _D. neglectum_ Massee, reported from Philadelphia, is said to
be a slender form of the present species. The figures of _D. clavus_ by
Albertini and Schweinitz are excellent, as also the description.

Not common. Pennsylvania, Ohio, Iowa.

10. DIDYMIUM NIGRIPES (_Link_) _Fries._

PLATE VII., Figs. 2, 2 _a_, 2 _b_.

1809. _Physarum nigripes_ Link, _Obs. Diss._, I., p. 27.
1818. _Physarum microcarpon_ Fr., _Sym. Gast._, p. 23.
1829. _Didymium nigripes_ (Link) Fr., _Syst. Myc._, III., p. 119.
1875. _Didymium microcarpon_ (Fr.) Rost., _Mon._, p. 157.
1896. _Didymium microcarpon_ Fr., Morg., _Jour. Cin. Soc._, p. 61.

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.
Und