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.
Lime (fruit)
Note that the tree species known in Britain as lime trees ("Tilia" sp.), called "linden" in other dialects of English, are broadleaf temperate plants unrelated to the citrus fruits.