Found on a rock in the tide pools of Fitzgerald Marine Reserve! Its shape reminds me of a trilobite!
Marine Corps Recruit Deport (MCRD), Recreation area/marina, rip-rap shoreline with mixed rock/sand substrate, collection site adjacent to concrete storm drain?, south side on rock from intertidal zone collected as afternoon tide was rising, rock collected from water depth approx. -= .3 meter, associated with macro fauna tunicate Styela plicata - several specimens found and photographed dorsal and ventral views - living animal was removed from a few shells to visualize the "cup" inside the shell, exterior of some shells with very short spine tubes. several shells with one or two smaller individuals stack on dorsum of larger individual - likely male(s) on female shell,
Ken-ichi pointed out this bonus guy on the side of the Brown Turban Snail. Superb cool.