For accurate identification, it is necessary to ensure that your material is mature. Immature slime moulds are soft, slimy and easily deformed by touching with a finger tip. When mature, they become dry, more rigid, often crisp, crumbly or flaky, and you will probably be able to see a powdering of spores on your sample or on the substrate. The mature sporocarps may also be a different colour from the immature slime. 

Slime moulds belong to a number of different families which can be fairly readily distinguished by easily observed features:

Spores born internally or externally?

Almost all myxomycetes produce their spores inside a sporangium or other fruiting structure (such as a plasmodiocarp or an aethalium), the only exception is the genus  Ceratiomyxa in the Ceratiomyxales, which carries its white or pale spores on the outer surface of the sporocarp. 

Ceratiomyxa fruticulosa              

 Ceratiomyxa porioides

Presence of a capillitium?

Most myxomycetes have a system of filaments (the capillitium) within the sporocarp which help to support and disperse the spores. The filaments may be branched or not, brightly coloured, dark, or inconspicuous, with or without lime inclusions, but in one group there is no such system. This group is the Liceales, a diverse group including  Cribraria, which has sporangia resembling tiny wire-netting cages, Licea, a genus of minute species usually only found in bark cultures, and Enteridiaceae (Reticulariaceae), which includes Enteridium (Reticularia), Lycogala and Tubifera amongst others. These form large aethalia, fruit bodies formed from the merged mass of a large aggregate of sporangia. Some of these may appear to have a capillitium, but this pseudocapillitium is merely the remnants of what would have been individual sporangial walls. Spores may be yellow, orange, pink, red, whitish, grey ochraceous or violet depending on species. Lime is never present.

Young  Cribraria argillacea sporangia. 

Aethalium of  Enteridium lycoperdon

Younger and more mature developments of  Tubifera ferruginosa.

Presence or absence of lime?

This can often be seen as a white (sometimes pale coloured) flaky, granular or crystalline layer on the surface, but it may be hidden within the sporocarps. A small drop of vinegar or other dilute acid will quickly produce effervescence (bubbles) from your specimen, easily seen with a hand lens if not with the naked eye.

The Physarales includes all the families which contain lime. It is present in the peridium and capillitium in the Physaraceae, which includes Physarum, Craterium, Leocarpus, Fuligo and Badhamia, but only in the peridium in the Didymiaceae, which includes Didymium, Diderma, Diachea, and Lepidoderma. Spores are very dark purple or black.

Physarum leucophaeum 

Craterium leucocephalum

Fuligo septica  with yellow lime.  

The dark spore mass can be seen within.

Lepidoderma tigrinum. The typical limy plates on the  peridium are poorly developed in this example.

This leaves two large groups and one smaller one, without lime in the sporocarp, which can be distinguished by spore colour and capillitium. The Stemonitales have very dark or black spores, being distinguished from the Physarales by the absence of lime. Many of these, like Lamproderma, Comatricha and Stemonitis have stalked sporangia with a well developed, often dendroid capillitium, with the appearance of tiny trees or loofahs. The stalk often extends into the sporangium to form a columella, to which the capillitium is attached.

Stemonitis fusca  with brown spore mass.

 Group of developing sporangia of  Stemonitis.

The Trichiales have brightly coloured spores and a capillitium which often expands on maturity to help disperse the spores. The capillitium is tubular, often branched, and may be smooth but is more often ornamented with warts, spines, cogs or spirals. It may be free or attached to the base of the sporangium, but there is never a columella. Sporangia may be stalked, sessile, or plasmodiocarpous.

Genera include  Trichia, Metatrichia, Hemitrichia, Arcyria, Oligonema and Perichaena.

Stalked sporangia of  Trichia decipiens

Sessile (stalkless) sporangia of  Trichia varia.

 Immature  Trichia decipiens  

Young  Trichia botrytis sporangia

Spores and capillitium of  Trichia spp.

Sessile sporangia of  Perichaena depressa

Stalked sporangia of  Hemitrichia calyculata  The upper part of the peridium has peeled away. showing expanded capillitium.

This leaves the Echinosteliales, a group of very small, simple species, which was probably basal to the other groups. Genera include Echinostelium, Clastoderma, and Barbeyella which may have given rise to the Stemonitales, and Protophysarum, which appears to be basal to the Physarales.

Echinostelium elastichon

Echinostelium minutum

Species of Echinostelium  consist of little more than a simple tubular stalk up to 1.5mm tall, one or two capillitial threads, and a few pale spores.

Identification of Slime Moulds