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.
Corsican citron
For a short period of time Genoese merchants, who always supplied fruit for the Jewish ritual of Etrog, used to ship also some amount of this Corsican variety, while there was not enough available from Diamante. This tradition terminated due to competition with the Greek citron which was considered to be of extraordinary beauty.