aside: me after reading the headline: Shouldn't that be Octopii?
But no!
"The plural form octopii is doubly incorrect. Firstly, octopus derives from Greek, not Latin; its etymologically-consistent plural form is octopodes. Secondly, even if octopus were a second-declension Latin noun, the plural form would be octopi; in the correct plurals radii and gladii, with which octopii is analogous, the first ‘i’s are part of the words’ stems (radi- and gladi-), and not their case endings — for octopii to be the plural, *octopius would need to be the singular."
But no! "The plural form octopii is doubly incorrect. Firstly, octopus derives from Greek, not Latin; its etymologically-consistent plural form is octopodes. Secondly, even if octopus were a second-declension Latin noun, the plural form would be octopi; in the correct plurals radii and gladii, with which octopii is analogous, the first ‘i’s are part of the words’ stems (radi- and gladi-), and not their case endings — for octopii to be the plural, *octopius would need to be the singular."
Thanks wikipedia.
TIL!