Thomas and Mary will most probably have been buried in the old churchyard at Cambusnethan. There is a very large family plot with three separate headstones on it. I believe this is where they are. 2 of the stones are identical and I think the easiest way to do this is to give you the transcriptions from each stone. The relationship is usually to the first name on the stone unless otherwise stated.
"1868 Erected by the Rev James Burns, minister at Nairn in memory of his father John who died 28th March 1866 aged 86 years. His mother Sarah Smith died 31st January 1853 aged 68 years. His brother William died 25th March 1867 aged 57 years. His brother John died 10th October 1836 aged 19 years. His sister Janet died 30th December 1836 aged 24 years. His nephew Archibald Bell died 3rd October 1843 aged 4 years & 6 months. His sister Sarah died 29th July 1895 aged 75 years. His sister Marion died 28th January 1896 aged 73 years. The above Rev James Burns died 17th May 1897 aged 72 years and is interred at Nairn".
next stone -
"1891 Martin Bell died 26th October 1874 aged 68 years. His wife Mary Burns died 7th August 1888 aged 73 years. Their son Archibald died 3rd October 1843 aged 4 years & 6 months. Their daughter Marion died 26th November 19(. . ) aged 73 years (wife of Matthew Cleland, Cambusnethan)".
These two headstones match exactly in design.
next stone -
"Erected by William Burns in memory of his wife Janet Yetts who died 5th December 1808 aged 27 years". On the reverse of this stone is inscribed -
"Thomas Burns died 17th November 1830 aged 36 years, husband of Marion Russell. Their daughter Susan died 1828. Their daughter Jean died 1838. Their daughter Marion died 1845".
OK, proof that this is the right family.
Extracts from the gravediggers Day Book -
JOHN BURNS, joiner proprietor, Bonkle, aged 86 years, widower.
Parents - Thomas Burns & Mary Gardener.
Buried in the old churchyard on 3rd April 1866.
(These details match with the John Burns on the first headstone).
WILLIAM SMITH, proprietor, Bonkle, aged 57 years, unmarried.
Parents - John Burns & Sarah Smith.
Interred in the old churchyard on 29th March 1867.
The Plot Book for the churchyard only begins about 1876, this is the information for the above plot, which seems to have been sub-divided over the years -
Owners (in Victorian times) Thomas Burns & Rev James Burns.
In the south lair -
Marion Cleland, aged 73 years, interred 28th November 1918.
In the centre lair -
Marion Bell, aged 74 years, 10th August 1888.
In the north lair -
Sarah Burns, aged 75 years, 31st July 1895.
For the William burns & Janet Yetts stone -
Owner - Marion Russell.
In the centre plot -
Catherine Burns, aged 70 years, 3rd September 1894.
Other extracts from the Day Book -
ISABELLA BURNS, Blair's Orchard, aged 74 years, widow.
Parents - Thomas Burns & Mary Gardener.
Old churchyard on 3rd January 1861.
(Do you know if she married James Martin?)
JAMES BURNS, joiner, Newmains, aged 81 years, unmarried.
Parents - Thomas Burns & (. . . . . ) Gardner.
Old churchyard on 17th January 1877.
JANET SMITH, housewife, Cocket Hat, aged 85 years, widow.
Parents - Thomas Burns & Mary Gardner.
Old churchyard on 8th March 1879.
LILIAS DALZIEL, Wishaw, aged 91 years, widow.
Parents - Thomas Burns & Mary Gardner.
Old churchyard on 2nd July 1879.
MARION BURNS, housewife, Glasgow, aged 80 years, widow.
Parents - Thomas Burns & Marion Russell.
Old churchyard on 7th June 1869.
SARAH SMITH MARSHALL, Newmains, aged 74 years, widow.
Parents - Martin Bell & Mary Burns.
Old churchyard on 21st February 1911.
Janet Smith who lived at the Cocket Hat (an area in Wishaw at the corner of Kirk Road & Coltness Road) who was buried 8th March 1879, is in a Smith family plot. The Plot Book gives the following information -
In the centre lair -
Janet Smith, aged 85 years, 8th March 1879.
Mary Smith, aged 64 years, 1st February 1892.
In the North lair -
William Smith, aged 37 years, 14th July 1892.
I hope you can make sense of all this?
Here is a picture of the Burns, Yetts and Russell stone, I only have the one side. If the weather is clear at the weekend I'll try and take another picture.