Desert Recluse - Loxosceles sp.
We found two of these under a car seat that had been dumped in the desert. The other one ran down a hole in the sand. I think this may be Loxosceles deserta, but Steve Lew's list also includes Loxosceles martha & Loxosceles palma in Riverside County.
BugGuide info page found here: bugguide.net/node/view/31531.