No, that's for -ius, which "radius" is and "octopus" is not. "Octopus" takes one I to go plural - "octopi."
Which is just what I said.
Anyway, according to Dictionary.com, either is correct. So I withdraw my complaint, if the original poster withdraws his.
But the plural of "virus" is NOT "virii", as I have seen elsewhere. Consistency is not a virtue of the language....