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
Ponderosa lemon is less cold hardy than a true lemon. It bears medium to large fruit that have a thick and bumpy rind. The fruits are seedy, and while they look similar to a citron, they taste like a lemon.