Question: A friend who has a collection of old rosary beads, showed a couple of them to me going back to his grandparents childhood on which there was a skull and crossbones at the foot of the cross. What is the significance of the skull and crossbones in Catholic symbolism?
Answer: I believe the answer to your question about the meaning of the skull and crossbones image below the cruxifix goes something like this: there is an ancient legend that Adam & Eve died at the place-of-the-skull which in Hebrew is called Golgotha and we know as Calvary.
In popular tradition it was of mound of rock that was shaped something like a skull and this is where Jesus was crucified just outside the walls of Jerusalem around the year 30 A.D.a