Citron
There are also a number of citron hybrids; for example, ponderosa lemon, the lumia and rhobs el Arsa are known citron hybrids, some are claiming that even the Florentine citron is not pure citron, but a citron hybrid.
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.