Rhobs el Arsa
The Rhobs al Arsa has long been viewed as a citron hybrid, though a different cultivars have been suggested for the parent, for example, Moroccan citron as a male parent and sour orange as the female parent, citron or lemon and a mandarin., or citron and the lime. Detailed genomic analysis has shown Rhobs al Arsa to be highly similar to several cultivars of "Citrus limetta", the limetta or Persian sweet lime, and likely shared a common origin with them, arising from a cross between citron ("Citrus medica") and sour orange ("Citrus" × "aurantium"). The common lemon arose from a distinct hybridization event involving the same two species.
Citron
Citron was also described by Pliny the Elder, who called it "nata Assyria malus". The following is from his book Natural History:
There is another tree also with the same name of "citrus," and bears a fruit that is held by some persons in particular dislike for its smell and remarkable bitterness; while, on the other hand, there are some who esteem it very highly. This tree is used as an ornament to houses; it requires, however, no further description.