Call the cops. It may be someone that they gave up looking for. Families of murder victims deserve closure. Quit digging and call the cops. The more you dig, the more you screw up the evidence.
You *don't* get to keep the skull to put on your bookshelf. That's somebody's mom, dad, kid or grandparent. Would you want a member of your family's skull kept for a trophy? Naaaw.
Do the right thing and call it in.
Jan