Institutio Oratoria

Their proceedings are just as arbitrary as if they were to substitute an o in the genitives of sulpur and guttur on the analogy of eboris and roboris. Thus Antonius Gnipho while admitting robur, ebur and even marmur to be correct, would have their plurals to be ebura, robura and marnura.

If they would only pay attention to the affinities existing between letters, they would realize that robur makes its genitive roboris in precisely the same way that limes, miles, iudex and uindex make their genitives militis, limitis, iudicis and uindicis, not to mention other words to which I have already referred.