pileus

Photo of a pileus or mushroom cap

In mycology, the pileus (pl.: pilei) is the cap or cap-like part of a basidiocarp or ascocarp (fungal fruiting body) that supports a spore-bearing surface, the hymenium. The hymenium (hymenophore) may consist of lamellae, tubes, or teeth, on the underside of the pileus. A pileus is characteristic of agarics, boletes, some polypores, tooth fungi, and some ascomycetes.