Those look like your common brown garden mushrooms, I wouldn't think you were growing portabellos unless you had a horse manure casing, which brings me to the next question, did you recently lay down a layer of some kind of poop?
No poop. But i did layer lots of fresh unbroken kitchen waste and banana leaves (which i read is a great substrate for Oysters). I went against the advice of not composting banana leaves as they take a long time to breakdown and i was thinking maybe mushrooms would probably do great. I usually collect kitchen waste in 5 gal buckets (as they get full) i dump it right in to the bed and bury it (usually work in sections) and let it break down like that rather than in the compost bin. What i throw in the compost bin is usually all the really fine stuff (like Juicing leftovers), coffee grounds, and Oyster mycelium i grow in cardboard and paper waste etc.
I tend to steer away from agarics because theres been a lot of accidental poisonings in so cal over the last few years.
It reminds me of a few in the san bernardino/riverside co, It could be a horse mushroom(I really don't think it is) it could be bisporus (very possibly), it looks like a lot of the fungus that I've found in the area, The good thing is, if you're wrong and it's californicus, it won't kill you straight off and always check new flushes against the definition. But like it was said using KOH is a good way to arrive at an ID.
2nd picture shows some showing up from the soil in early January. They were flushing from beneath the soil level. Some showed up in the Nov the previous year and now in March. Seems to be spreading further out also. Plants in this bed are growing absolutely amazing.
You could possibly send a few pics to Michael Kuo at
http://www.mushroomexpert.com/
in fact
you might want to check your info against this page
http://www.mushroomexpert.com/agaricus_bisporus.html
and definitely check your spores.
Gaston
PS. you wouldn't mind sharing some spawn for compost purposes in the future would you?