The false parasol (Chlorophyllum molybdites) is a very common and very toxic mushroom, often found in Southern California lawns. Because its appearance is similar to the edible parasol mushroom, and because it is so often found in lawns, it is one of the most commonly-consumed poisonous mushrooms in North America. Apparently it will not kill you if eaten, but will make you very uncomfortably sick, with vomiting and diarrhea, so is best avoided altogether.
Interestingly, when I picked the mushroom, it looked like this,
and by the next morning the cap had opened and spread to this,
The brown scales had also begun to flake off -- it often looks like this while growing.
The ruler I used was not the best for this lighting, obviously -- this particular mushroom is about 4 inches (10cm) long, but can be found larger or smaller.
A distinctive characteristic of the false parasol is the greenish spores, which are beginning to be apparent in the photo, and came out quite clearly in the spore print I made -- the name "chlorophyllum" comes from the Latin chloro, green, + phyllum, gills.
For further reference, and other pictures to help you identify this toxic mushroom, see the Wikipedia article "Chlorophyllum molybdites", Tom Volk's "Fungus of the month" for August 1999, the C. molybdites pages at California Fungi and Urban Mushrooms.
