Peridium

The peridium is the protective layer that encloses a mass of spores in fungi.

Depending on the species, the peridium may vary from being paper-thin to thick and rubbery or even hard.

[2] In the simplest subterranean forms, the peridium remains closed until the spores are mature, and even then shows no special arrangement for dehiscence or opening, but has to decay before the spores are liberated.

In contrast, the inner layer remains continuous and smooth to preserve the spores.

[5][6] This lone peridium is generally rigid and rindlike; it is white when sectioned but pink if fresh.

The outer layer of this bird's nest fungus ( Crucibulum laeve ) is the peridium.
Internal surface of the peridium of the rare myxomycete Tubifera dudkae is covered with folds