St Ninians Priory - Whithorn

Apparently.........

If the crossed bones are underneath the skull, there are several possible theories:
Relating to the Knights Templar, as they used the insignia to deter enemies. On crucifixes, sometimes the crossed bones are under the cross to denote Golgotha or the place of skulls where the crucifixion took place. Connected to some European churches, where the S&C displayed at entrances.
In 1700's the southern Scots used the S&C on their headstones denoting death.
It could be that the grave possible contains the remains of a Freemason, and the Freemasons used this symbol to denote a Master Mason.
Or It could also mark the grave of a covenanter, perhaps martyred for the cause in scotland.
The headstone faces in the wrong direction, suggesting an un-natural ending for the incumbent perhaps.
The hour glass suggests our time on earth is limited.