Nopalea dejecta, Spiny Nopal, belonging to the same family as the Prickly Pear
A stamp depicting cactus issued by the Principality of Monaco in 1960.
Apparently this plant is naturalized in so many parts of the world that it is not known where it originated
The article I am recommending does not feature a Prickly Pear but it does give a description on the Jardin Exotique de Monaco.
THE MEDITERRANEAN GARDEN number 79, January 2015
THE 2014 AGM:
THE MAIN PROGRAMME, MENTON, COTE D’AZUR
By Edith Haeuser
For photographs of some of these gardens, see
THE MEDITERRANAN GARDEN SOCIETY website.
http://www.mediterraneangardensociety.org/79-agm.html