When young, the poisonous Coprinopsis picacea
can have a similar appearance. It differs in that cap has a glossy, dark brown base color overlain with white fibrils. The dark base color becomes apparent in maturity, when the white fibrils become patchy. The inedible Coprinopsis lagopus
has a similarly elongated cap but differs in that it is much smaller (under 4 cm broad when fully expanded). It has a very delicate fruiting body that lasts only a few hours before dissolving into black ink. A number of other poisonous or inedible white mushrooms that have warty or scaly caps are superficially similar. They differ in that they do not turn inky/black when old, and/or do not have a blackish spore deposit.
See also Coprinopsis atramentaria