Something showed up on my lawn this week that I wasn't happy to see at all - the Green-spored Lepiota. Or in mycological world known as Chlorophyllum molybdites. Very poisonous to both human and animals and can easily kill cats or dogs if swallowed. Another one to watch out for is Death Cap. You can find more details on the websites like this: http://americanmushrooms.com/deathcap.htm
This is how it looks when it's young Lepiota just starting to open up.
Lepiotas have very distinctive pattern of spots on its caps.
As you can see on the underneath of it it has a ring up close to the cap - another distinctive trait for it. Mature mushroom has green spores which also help identify it. If you have it on your lawn - destroy it and keep pets away from the area because even spores are poisonous and a lick of them can cause serious damage.