Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
Corsican citron
The tree produces large fruit, ellipsoid to very slightly obovate; basal area slightly depressed and radially furrowed; apical nipple suppressed or indistinct. Color lemon-yellow when ripe. Rind very thick and fleshy, sweet with some bitter after-taste; surface rather rough, bumpy, and commonly somewhat ribbed. Flesh crisp and solid; lacking in juice; flavor sweet without acid. Seeds white yellowish. This giant citron can measure up to 25 cm in length and weigh up to 4 kg.