Collection location: Santa Clara Co., California, USA [Click for map]
tight cluster of white mushrooms. The mushrooms on the left show the bottoms. Found growing in mulch of wood chips from a conifer tree.
The second image shows the cross section of two fungi.
I essentially agree with Joshua, but I wouldn’t call them S. citrina since that species is actually pretty distinctive. It looks more like the S. cepa/areolatum/laeve group, but you’ll have a hard time deciding between those with specimens that are this young. Additional things to look for are staining reactions when you scratch the surface and if you have access to a microscope, look at the spores.
The purple to blackish colors of the gleba are indicative of a sceleroderma… As far as a species name goes it is tricky… the catch all name is citrina though… Did these have a large underground base?
If you still have them, cut one in half and send a photo.