I have this listed as Prairie Bishop, Bifora Americana. It blooms in May, June, July.... the flower heads are very delicate and the foliage is also delicate, dill-like and has a slight carrot scent, I think. Some of the flower heads are flat but some are more rounded.

close up view of flower

even closer

Your plant looks like it has smaller and more numerous flowers than the pictures I can find online. The leaves also look different than in the thumbnail at the following link.
Try this link you can click on the thumbnail for large picture.
http://www.noble.org/webapps/p...

I think that is why I never have confirmed the name of this wildflower. it grows in our open pasture in full sun.

I think it could be Atrema amrericanum, aka Bifora americana.
I posted some recently on the database, but the ones I saw on the prairie were smaller because they were growing on chaliche soil.

Could it be Daucus carota (wild carrot), or Queen Anne's Lace? We had it growing all over our front garden when we first moved here.

Frostweed, thanks for digging up this old post. I have this plant listed under here, what do you think?

I think you have it right Christine, I don't know what I was thinking, I should have read the entire thread.