Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
Siracusa Lemon
The European Union has nine (PGI) denominations for lemons: six of these are from Italy, one from France, one from Spain, and one from Portugal. Other than the "Siracusa lemon", the Italian denominations are the "Limone di Sorrento", the "Limone Costa d'Amalfi", the "Interdonato di Messina", the "Femminello del Gargano", and the "Limone di Rocca Imperiale". The French denomination is the "Citron de Menton". The Spanish denomination is the "Citricos Valencianos", while in Portugal the "Citrinos do Algarve" is protected.