Aethalium white or cream-colored, of variable size and shape, half-an-inch to three inches in length and half as thick, the component sporangia resting upon a common hypothallus and protected by a more or less deciduous calcareous porous cortex; peridial walls thin, and where exposed iridescent, generally whitened by a thin coating of lime crystals; capillitium scanty, of simple, mostly dark-colored, slightly anastomosing threads; columella indefinite or none; hypothallus white, spongy; spore-mass black, spores violaceous, exceedingly rough, large, 12-15 mu. Very common in all the eastern United States and the Mississippi valley, south to Texas. The plasmodium is dull white, of the consistence of cream, and is often met with in quantity on beds of decaying leaves in the woods. In fruiting the plasmodium ascends preferably living 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_