Hi kassie and osprey,
Some thoughts.
Lots of wonderful information has been provided - and may well turn out to be totally relevant.
But, going back to basics ...
There aren't really a lot of proven facts on which to trace Catherine backwards, are there (sorry to be a wet blanket!).
*Catherine Howell VENABLES (the correct Cath as she is described as wife of William VENABLES carpenter) died in Cardiff on 15 Nov 1910; there was an inquest held 16 Nov 1910.
Possible lines of inquiry (through the local records office?) might be - has the inquest report survived; is there a newspaper report of the inquest; is there a death notice/obituary?
*There was no middle name or initial on her marriage cert or on the 1881, 1891 or 1901 censuses.
*Catherine's age varied (marriage cert, censuses, death cert) - circa 1850 +/- 2
Catherine was older than her husband, William Edmund VENABLES, and might have slightly understated her age.
*On her 1878 Swansea marriage cert (Cath's residence 13 Park St), Catherine's father was listed as Thomas THOMAS, Harbourmaster.
No Thomas THOMAS, Harbourmaster can be found. Was Catherine simply "gilding the lily" as to his occupation? Another question might be whether her father really was a Thomas THOMAS (people have been known to invent a father on a marriage cert.) Her mother's name is not known although there are some possible/likely census entries for a Thomas THOMAS, wife Margaret.
*In the 1881, 1891 and 1901 censuses, Catherine's birthplace was always given as Port Talbot.
There is the following birth - Catharine (sic) Howell THOMAS, b Neath (this included Port Talbot), Dec quarter 1849, Volume 26, Page 455
If this birth cert were obtained, it would show the name and occupation of the father, the name and maiden surname of the mother, and the address where the birth took place (especially whether it was in Port Talbot).
This could help in tracing this Cath THOMAS in the 1851 census and then possibly in 1861 and 1871; this might well help to prove whether or not she is the Cath who m VENABLES.
So, for what they are worth, those are my thoughts i.e. get the birth cert; look for any Cath THOMAS, b Port Talbot, age 1, in 1851 (sorry I can't help); chase up the inquest/newspapers.
I'm hoping that Cath's Dad is Thomas THOMAS, Mariner/Mate, but at this stage that's a bit of a leap of faith.
Cheers,
JAP
PS: Kassie, Do you have Cath's occupation from her marriage cert? Might help in finding her in 1871?