Found under a rock, riparian forest, mostly oak and toyon with scattered foothill pine. I was going to call this H. arrosa, but Roth & Sadeghian don't list it for Solano County, which leaves H. nickliniana (wrong sculpture, open umbilicus), H. contracostae, and H. diabloensis. H. diabloensis seems to have oblique ridges in its sculpture and is supposed to be more tightly coiled, with 6-7 whorls, whereas I count 5 whorls here, so I'm going with H. contracostae.