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.
Moroccan citron
Chapot was probably first to describe this variety of citron in detail, along with illustrations of many forms of the fruit, and all the properties of the shrub, leaves and blossoms. He also mentions that the true citron of Morocco, which is traditionally only grown in the region of Assads, for the sake of the mitzva of four species, and is oblong, acidless and rather dry, is much different than the rounded citron hybrid, Rhobs el Arsa, that is more commonly grown for food in the entire country of Morocco, and its taste is acidic and fruity.