Hmmm, very hard to read.
Checking back to the indexed names on
www.gravestonephotos.com they only list two people as named on the headstone, with no reference to a death date for Thomasin. This only means the volunteer who transcribed it, couldn't read it or didn't notice

But, there is something in the inscription, at the bottom of the headstone, which says:
ALSO __________of the above_____died March(?)_________ It looks a bit like daughter, and the name is hard to decipher, though I think it's too short to be Thomasin.
Did they have any daughters?
Here's what I can make out:
William
The Beloved husband of
Thomasin Geen
who entered into his rest
_________1874 aged ___________years
Blest___ (can't read the verse at all)
ALSO___________Geen
daughter(?) of the above
Died March (?) ___________________
I would still be inclined to go back to the website and ask for further help from the local volunteer. I can do it for you if you like?
Caroline.