Panaeolus olivaceus

Panaeolus olivaceus is a widely distributed, rarely identified, little brown mushroom that contains the hallucinogen psilocybin. It is often mistaken for Panaeolus foenisecii and is distinguished by its black spore print and darker gill coloration when mature alongside a slightly thicker stem. It is even more easily mistaken for Panaeolus cinctulus or Panaeolus fimicola and can be distinguished from them both by its slightly roughened spores. It is also easily confused with Panaeolina castaneifolia, a species which has spores that are dark brown and significantly more roughened....read more on Wikipedia.