Look what came up in my shamrocks

Okay, it's a bit blurry. Blame the camera on my phone. It's Leucocoprinus birnbaumii, also known as the flowerpot parasol, plantpot dapperling, yellow parasol, yellow houseplant mushroom, lemon-yellow lepiota, or yellow pleated parasol, Their spores don't seem to be dangerous from anything I've read, but the mushroom itself can cause significant stomach distress if eaten. I went ahead and removed this after I took the picture. I've had to remove several, although this is the first one to mature past the button stage. I must admit, it is pretty. But as far as I'm concerned, mushrooms do not belong indoors unless you're planning on eating them.

These look absolutely Cthulhoid, especially when the plate is placed on its side like this

It's the The 63rd plate from Ernst Haeckel's Kunstformen der Natur, (1904) depicting Basimycetes, from WikiMedia Commons. You can click on the picture to make it bigger. They look like they came straight out of the Cthulhu Mythos.

I came across the illustration in Wired's The Magic of Mushroom Spores by Brandon Keim.

Of course, if you've read Brian Lumley's Necroscope books, you know that evil vampiric spores can be very, very dangerous.