Yep, as Lonnie�s link says pretty much.
Skull and long bones crossed were depicted or sculpted in catacombs, monasteries, churches, church and crypts, especially above the entrances to churches and graveyards. They served as a Memento Mori, meaning "remind yourself of your own death." It was a general warning against the sin of vanity, reminding by-passers of their mortality. Thus, it became at once a common symbol of death and decay and a warning against the vagaries of fortune, as well as a first hint of an emerging sense of democracy: in death, we are all equal.