Common Mushrooms and Portobello Mushrooms at Farmers Market in Union Square, NYC.
left:
When immature and white, this mushroom may be known as common mushroom, button mushroom, white mushroom, cultivated mushroom, table mushroom, and champignon mushroom.
right:
Agaricus bisporus is an edible basidiomycete mushroom native to grasslands in Europe and North America. It has two color states while immature —white and brown—both of which have various names. When mature, it is known as portobello mushroom, often shortened to just portobello.