I think it's a copepod, but I don't know the microcrustacea in general very well, so I've left the identification fairly broad.
Didn't get photo. Mushrooms were dried out, and I'd seen the same species there in fresh condition a couple weeks earlier - since I saw it on 4/20, I'm still noting it for the Tilden Bioblitz species list.
Physocarpus capitatus, how do you do. How have I lived 11 years in the Bay Area and never met you.
This is an ID based strictly on BugGuide data, where they claim there are only 3 North American species in this genus. BG range data suggests only M. eburneus and M. longicollis are the only species in CA, and M. eburneus has white markings on the elytra (e.g. http://bugguide.net/node/view/888019/bgimage), leaving me with a sweet, sweet species-level ID.
Adams goes says there are only 3 species of Cyclosa in CA, bless him, and this is the only one without tuburcles / bumps on the anterior end of the abdomen.
Found beneath a 9"x12" block of concrete, approx 2 meters from the shoulder of the road.