Coprinus is a large genera of Basidiomycetes who’s members are often referred to as “inky caps” due to the way the caps auto digest allowing the spores to be released in a gooey, inky, mass.  This species is one that is frequently found on dung, so it colonized and grew on our artificial agar poo-cube quite well! I believe the reason the mushroom grew from the bottom is that the poo-cube was seeded with the fungus while still in a beaker. When I removed the cube to place it on the Petri dish (up-side down), the mushroom primordial had already been formed and the growing stalk exited the cube via the slit at the bottom………the path of least resistance.