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
"This tree does not become particularly big. The leaves are smaller than those of other citrons, and serrated, oblong, pointed towards the front, mixed with many thorns. The bloom is small and reddish from outside. The fruit blossoms (- "the carpels") are not less oblong from the beginning, appearing as reddish and dark-green; thereafter they turn entirely green, and when they ripen, straw-yellow, remaining, however, rather small all the time and never growing to a proper size, like other kinds of Citron.