Fairy Ring Mushrooms

Princeton's mushroom community has responded well to recent rains. This one's looking like a Lepiota, a poisonous mushroom that can form rings in lawns as the mycelium radiates outward from its beginnings years back.

This one is growing in a remarkably straight line, which somewhat plausibly could be a remnant of a very large circle,

while this grouping is shaped in a half circle. According to this link, the darker grass at the leading edge of the circle is due to the grass's utilizing nitrogen made available by the fungi. Grass on the inside of the circle can be stunted.