The last county in Wales to be listed is the first (earliest) county in Wales to show the surname JONES. Of course, it had to be "John Jones"...:-). It is the county of Glamorgan.
John Jones was identified as the "son & exec." of Jankyn ap John [1515 - 1518] (bundle 421/ no. 72). The case had to do with the lease of "...1/2 the mills of Cardiff held by the s'd Jankyn of John Howell, baker".
Now Tacitus was the first to describe these folks in his "The Life of Agricola". He writes:
"The dark complexion of the Silures, their usually curly hair, and the fact that Spain is the opposite shore to them, are an evidence that Iberians of a former date crossed over and occupied these parts."
These Romans were to take control of this area by 84 AD.
It would seem that the legal system of England had already established its influence well before the Act of Union" 1536.
Well, say hello to John Jones, the first to take this surname in Wales.
The history of Glamorganshire can be found in Nicholas, Vol. I, pp. 458 - Vol.II - 648. The reference to Tacitus can be found in "The Historians of Ancient Rome", ed. Ronald Mellor, p.399.