As a boy in the late 1960’s, I remember finding large numbers of fossilized sand dollars in the hills in and around San Diego. Those areas are probably now under miles of housing tracts.
Out in the desert, off the 8 east, just past Jacumba but not quite to Ocotillo, there is/was a place up in the hills called Shell Canyon. I was just a little kid then, but I found a perfectly formed fossilized double clam shell there. Didn't know there were other places -- whereabouts in San Diego?