It 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.
My thoughts are that the grave possible contains the remains of a Freemason, and the Fremasons used this symbol to denote a Master Mason. The grave looks signoficant for its time, even though you cannot read the inscription and date, I guess it could have been someone important and Freemasonary was common in officials and wealthy folk.
http://www.cemetery-headstones.net/ Media URL: http://www.cemetery-headstones.net
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