Reference for Shevuot 11:2

והיכא שמעינן לרבי עקיבא זו למעלה מזו

But Sid hekal presents a difficulty: [with what shade can it combine? ] Obviously, then, our Mishnah [in making Sid secondary to Bahereth, and Kerum secondary to Se'eth] is not in accordance with R'Akiba's view. And where have we heard R'Akiba [enumerating the shades of leprosy] one above the other?

Jastrow