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.
Rangpur (fruit)
"Citrus" × "limonia" was introduced into Florida in the late nineteenth century by Reasoner Brothers of Oneco, who obtained seed from northwestern India. Though often described as a lemon hybrid, genomic analysis has shown it to be an F1 hybrid of a female citron ("Citrus medica") and a male mandarin orange ("Citrus reticulata").