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.
Citrus depressa
Citrus depressa ("Citrus" × "depressa", formerly "C. pectinifera", "shiikwaasa", "hirami remon" or "shīkuwāsā"), in English sometimes called shiikuwasha, shequasar, Taiwan tangerine, flat lemon, hirami lemon, or thin-skinned flat lemon, is a small, green citrus fruit rich in flavonoids and native to Taiwan and Okinawa, Japan.