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.
Citrus halimii
Citrus halimii or Mountain citron is a sour fruit variety belonging to the citrus papedas, not related to the true citron. It was first discovered and catalogued in 1973.