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.
Greek citron
Historically speaking, the citron is considered by numerous writers, to be introduced to Europe by the troops of Alexander. It was also described by Theophrastus, who succeeded Aristotle as the curator of the Botanical garden in Athens.