Spinellus fusiger, commonly known as the bonnet mould, is a species of fungus in the Zygomycota phylum. It is a pin mold that is characterized by erect sporangiophores (specialized hyphae that bear a sporangium) that are simple in structure, brown or yellowish-brown in color, and with branched aerial filaments that bear the zygospores. It grows as a parasitic mold on mushrooms, including several species from the genera Mycena, including M. haematopus, M. pura, M. epipterygia, M. leptocephala, and various Collybia species, such as C. alkalivirens, C. luteifolia, C. dryophila, and C. butyracea. It has also been found growing on agaric species in Amanita, Gymnopus, and Hygrophorus.