Blodyn, this is the front view of the flower that you said might be a buttercup. Do you still think so? Thanks :)

I would say it's definately a Ranunculus. Could you show a photo of the leaves ?

I didn't get a photo of the leaves...I think that that's tem in the blurry background :)

I still think it's a buttercup ( or what is known here as a buttercup - maybe in different countries plants have different names ) . Anyway I think it's Ranunculus acris - Meadow buttercup. The previous photo is exactly like one.
If you're still not sure can you take some photos of the leaves and it's habit of growth e.g. if it creeps along etc.

Blodyn- I found something online called a Marsh Marigold (Caltha palustris) here: http://mihummingbirdguy.blogspot.com/2008/04/earth-day-april-22-2008.html
I read that they're closely related to the buttercup. The center of this flower looks different to me than the buttercups. Buttercups have that starwberry looking center, right? Plus, this flower has very faint black lines that run through the petals and I don't see that in any of the (many ) pictures that I looked at online. I photographed this flower in San Francisco, so I'll have to wait till I'm back in that area to check out the leaves :) The picture of the Buttercup on page 3 is different than this one, it wasn't the same plant. They could be kissin' cousins though. I'm sorry to bother you and thanks for your help :)

It's no bother Tasteyg, I thought it was the same plant when you asked if I still thought it was a buttercup. I misunderstood, sorry. I still think the first photo is a buttercup, but the second, well now you've said it's a diferent plant I 'm not sure what it is.
The petals don't look wide enough for a marsh marigold, and it looks as if it has sepals in the photo, which a marsh marigold doesn't have.. I'll look at the website you've put above.

I found this
http://bayareawildflowers.com/files/Bay_Area_Plants.pdf
It's a photographic guide to wild plants in San Francisco Bay area. Maybe you'll find it there. I saw one which it could be but the writing is small. I don't know how to enlarge it. Mayby you can.

Thanks for the website Blodyn. I checked it out and didn't find it. I think that I'll give this one a rest for now. Thanks very much for your efforts, though :)