This week 30 to 50 mushrooms as tall as five inches have occupied the pile of horse manure Buck and I deposited in the garden a few weeks back. It's a black-spored species with a slender stem and a fairly narrowly dome-shaped, cream-colored cap that when moist becomes sticky with a clear slime. I think it's PANAEOLUS SEMIOVATUS, but won't swear by the name. If it has a common name I can't find it. It ought to be called horse-manure mushroom because it's certainly a manure-lover, and it prefers what issues from horses above all other animals. You can see the species at www.mykoweb.com/CAF/species/Panaeolus_semiovatus.html
There's enough of these mushrooms populating our heap to make a decent mess for eating, but I'm letting that meal pass. Sources disagree on whether the species is edible. Most discourage trying it and some say it's poisonous with hallucinogenic effects. It grows along my garden rows where I've strewn horse manure as a side-dressing. Its mycelium is working to break down the manure into nutrients usable by my crops, so I'm grateful for that help and don't mind overlooking its offer of an altered state of awareness, and maybe a busted liver.
California mushroom lovers are lucky to have the "Mushrooms of California" Web site, describing 437 species and with over 1800 photos. Since fungal spores travel long distances, even people far from California can find this site useful. There's a "simple key" for mushroom identification linked to from the sidebar on the left of the homepage at www.mykoweb.com/CAF/index.html