As I posted on the Wiktionary Tea Room:
Consulting Bergk's edition of Sappho, I have seem various instances of this genitive "Sapphonis" (e.g. «Sapphonis esse videtur») in the critical notes. This struck me as odd because I'd always known Sappho as a Greek name which as such was declined in Latin as in Greek. Tonight I opened the Sappho entry here, and I found confirmation of my knowledge, and no trace of the genitive Sapphonis. So I was wondering: is it a very late genitive of Sappho, or is there a whole other version of the name giving this genitive? And if the former, shouldn't we mention it in Sappho? And if the latter, do we have an entry covering that version?
Since I got no responses in over two weeks, I am cross-posting. Is it correct that, at least in Classical Latin, the Genitive form was the Greek form Sapphus? When did the genitive shift to Sapphonis, and why, if it's known? Is Sapphonis merely a form used in critical apparati?
Here is the result of Ngram:
It appears that Sapphus prevailed over Sapphonis almost always, except for that sudden incredible spike in Sapphonis around 1740 AD. Huh? What happened then?