1. Capillitium irregularly reticulate throughout; calcic nodes various _Physarum_ 2. Capillitium more regular, especially below, furcate; nodes fusoid _Tilmadoche_ SECTION I. PHYSARUM I. Fructification not stipitate, more or less plasmodiocarpous. 1. Peridium simple. _a._ Calcareous deposits yellow 1. _P. serpula_ _b._ Calcareous deposits reddish or orange 2. _P. lateritium_ _c._ Calcareous deposits white, peridium rugulose 3. _P. vernum_ 2. Peridium double. _a._ Fructification flatly compressed 4. _P. sinuosum_ _b._ Fructification less compressed, rounded. i. Outer peridium white 5. _P. bitectum_ ii. Outer peridium brown or brown-tinged 6. _P. bogoriense_ iii. Outer peridium yellow; capillitium yellow 7. _P. alpinum_ II. Fructification of sporangia more or less distinct. A. Sporangia sessile, globose, ovoid, reniform, etc. 1. Peridium double. _a._ Sporangia white, peridium testaceous. 8. _P. diderma_ _b._ Sporangia tinged with yellow. i. Sporangia as if interwoven, compressed 9. _P. contextum_ ii. Sporangia more nearly free, distinct. o Spores pale, inner peridium brittle 10. _P. conglomeratum_ oo Spores spinulose, dark violet 11. _P. mortoni_ _c._ Sporangia brown, dehiscence revolute 12. _P. brunneolum_ 2. Peridium simple, calcareous, flaky. _a._ Sporangia grey, plasmodiocarpous; spores dusky, 10-12 forms of 3 _b._ Sporangia grey, more or less dense; spores violet, 6-7 13. _P. cinereum_ _c._ Calcareous deposits yellow or greenish, spores 7-9 14. _P. virescens_ _d._ Sporangia rusty or reddish brown, more or less dense 15. _P. rubiginosum_ _e._ Sporangia minute, lignicolous, the fructification much extended upon a hypothallus, lime deposit tawny 16. _P. instratum_ _f._ Sporangia white, depressed, annulate, sometimes with short stipes 17. _P. megalosporum_ 3. Peridium simple, not flaky, small .2-.3 mm., heaped 18. _P. confertum_ B. Sporangia, at least some of them, stipitate. _a._ Sporangia columellate. i. Columella small, usually conical. O Sporangium yellow. o Columella white 19. _P. melleum_