g stems of
small bushes, herbaceous plants, or grasses, and forms the aethalium
around the stem some distance above the ground. The cortex varies in
amount, is also deciduous, so that weathered or imperfectly developed
forms probably represent the var. _S. cornuta_ Schum.

Two varieties of this species are recognized; the one from Bolivia, var.
_dictyospora_ described by Mr. R. E. Fries (_Arkiv. for Botanik_ Bd. 1,
p. 66) differs from the type chiefly in its finer capillitial threads
its darker spores with longer spines and fine reticulate sculpture; the
other from Colorado, var. _solida_ described by Professor Sturgis
differs, as the name implies, principally in its greater compactness and
slightly smaller calcareous crystals; a desert phase.

=2. Didymium= (_Schrad._) _Fr._

1797. _Didymium_ Schrad., _Nov. Gen. Plant._, p. 20, in part.
1829. _Didymium_ (Schrad.) Fr., _Syst. Myc._, III., p. 113.
1875. _Didymium_ (Schrad.) DeBy., Rost., _Versuch_, p. 13.

Sporangia distinct, stipitate, sessile or even plasmodiocarpous, never
aethalioid; the peridium thin, irregular in dehiscence, covered with a
more or less dense coating of calcareous crystals; columella more
frequently present; capillitium of delicate threads, simple or sparingly
branched, extending from the columella to the peridial wall.

The genus _Didymium_, as set up by Schrader _l. c._, included a number
of species now assigned to _Diderma_, _Lepidoderma_ or _Lamproderma_.
Fries set out the didermas; DeBary and Rostafinski completed the
revision by setting out the remaining alien forms.

The genus is among Myxomycetes instantly recognized by the peculiar form
of its calcareous deposits, stellate crystals coating, or merely
frosting, usually distinct sporangia.

=Key to the Species of Didymium=

1. Lime-crystals merely whitening the peridial wall.

A. Fructification plasmodiocarpous.

_a._ White.

O Capillitium with adherent vesicles 1. _D. complanatum_

OO Capillitium simple 2. _D. anellus_

OOO Capillitium much combined; spores
10-13 mu 3. _D. wilczekii_

OOOO Capillitium crystal-bearing 18_a_. _D. anomalum_

_b._ Yellow or tawny 4. _D. fulvum_

B. Fructification normally of distinct sporangia.

_a._ Sporangia sessile or nearly so; outer