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.
Citrumelo
Trifoliate orange, according to Swingle, belongs to a citrus related genera, called Poncirus, while grapefruit equivocally belongs to the genus citrus, hence the botanical name "X citroncirus" is a hybrid genus, derived from citrus and poncirus.