The detailed birth records from Paris seem very recently to have appeared on the internet.
Irza Thomas, the mother of Lionel Britton, was born at 168 Rue St Denis, in what was then the 1st Arrondissement of Paris, on 21 May 1866.
The above picture is a close-up of part of the entry. It shows that the registrar was perhaps slightly pickled at the time, as he makes her 'fils' of her parents while conceding that she is of 'sexe feminin'. The recorded middle name Marie is not otherwise known, and the records available elsewhere state consistently that she was Irza Vivian Geraldine. Perhaps he mixed up her middle name with that of her mother, Antoinette, who is more familiar to us as Marie Antoinette.