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.
Ponderosa lemon
The ponderosa lemon ("Citrus " x "pyriformis") is a citrus hybrid of a pomelo and a citron. It is not the same as the 'Yuma Ponderosa' lemon pomello hybrid, that is used as citrus rootstock.