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 calcareous wall conspicuously developed 5. _D. crustaceum_ _b._ Sporangia plainly stipitate. i. Peridium much depressed; umbilicate below. O Stipe white 6. _D. squamulosum_ OO Stipe black. + Larger, about 7.5-1 mm. 7. _D. melanospermum_ ++ Small, about .5 mm. 8. _D. minus_ +++ Sporangia discoid 9. _D. clavus_ ii. Peridium small, globose. O Stipe dark brown or black; columella dark, obsolete or none. 10. _D. nigripes_ OO Stipe generally paler, of various tints of brown, orange, etc. + Columella pale or white, nearly smooth 11. _D. xanthopus_ ++ Columella, yellow, discoid, rough 12. _D. eximium_ iii. Peridium turbinate, columella hemispheric 13. _D. trochus_ iv. Peridium annulate 14. _D. annulatum_ 2. Calcareous crystals forming a distinct crust. A. Fructification wholly plasmodiocarpous 15. _D. dubium_ B. Sporangia ill-defined, sessile, plasmodiocarpous. _a._ Spores generally nearly smooth 16. _D. difforme_ _b._ Spores very rough, obscurely banded 17. _D. quitense_ EXTRA-LIMITAL _a._ Sporangia discoid, spores reticulate 18. _D. intermedium_ _b._ Stipe, columella, peridium, orange-brown 19. _D. leoninum_ 1. DIDYMIUM COMPLANATUM (_Batsch_) _Rost._ PLATE XVI., Fig. 8. 1786. _Lycoperdon complanatum_ Batsch, _Elench. Fung._, I., p. 251. 1829. _Didymium serpula_ Fr., _Syst. Myc._, III., p. 126, Rost., _App._, p. 21. 1875. _Didymium complanatum_ (Batsch), Rost., _Mon._, p. 151. 1899. _Didymium complanatum_ (Batsch) R., Macbr., _N. A. S._, p. 85. 1911. _Didymium complanatum_ Rost., List., _Mycetozoa, 2nd ed._, p. 127. Fructification plasmodiocarpous, creeping, flattened, vein-like, annulate or reticulate, the dark-colored peridium covered with white, but not numerous crystals; hypothallus none; columella none; capillitium much branched, violaceous threads combined to form a rather dense net which bears numerous, peculiar, rounded vesicles, yellowish in color, 30-50 mu in diameter; spores minutely warted, 7-9 mu, violaceous-brown.