Fruiting body a stalked or sessile sporangium or (less commonly) a plasmodiocarp. Stalk, when present, stout, calcareous. Hypothallus inconspicuous to extensive and then white and strongly calcareous. Peridium typically consisting of two layers, but in some species apparently made up of one or three layers, the outer layer calcareous or cartilaginous, the inner layer membranous, and the middle layer, when present, calcareous, the lime amorphous or crystalline. Columella usually conspicuous, sometimes reduced to a thickened, intrusive, dome like base. Capillitium thread-like, branching and anastomosing, limeless or sometimes replaced by limy columns. Spores dark brown or black in mass.
More than 60 species have been described for the genus Diderma (Lado 2001), 14 of which have been recorded from New Zealand. As the name itself implies, species assigned to this genus usually appear to have a peridium that consists of two layers. This feature is often readily apparent and thus useful in making a tentative field identification.