Collection location: Howarth Park, Santa Rosa, Sonoma Co., California, USA [Click for map]
In the (crummy) photo of the dissected one, the peridium seems to have stained reddish. The gleba has a yellow cast, which doesn’t show up all that well. I’m not seeing the embossed effect on the surface that is quite distinct on the examples of S. citrina that Nathan and Darvin posted. I ruled out S. geaster based on the not very thick peridium.
May 21: I’ve added two photos showing the surface of the peridium beginning to crack. Because of this, and using the key that Darvin suggested in his comment I feel pretty certain that these are S. cepa.
May 30: New image added, showing black/purple gleba visible as surface cracks.
June 4: Final image shows organism well and truly cracked open.
There are only four species in our area that bruise red.
Two have scales and two have a have a smooth peridium.
Try the Pacific Northwest Key Council Sclerodermataceae Key.
Although California was excluded from the Council, most of their keys work very well for northern California and all of them are more complete, have updated nomenclature and contain insights missing from Mushrooms Demystified. They are the best keys available!
All of the PNKC keys are free and can be downloaded at http://www.svims.ca/council/keys.htm.