polypodeum, with an e

It is worth knowing why, because this ending is not the usual botanical one. Most genus names ending in -ium got there by suffix, but polypodium got there by inheritance. Greek podion became Latin podium, an ordinary noun meaning a raised base or platform, still in use in English for the thing a conductor stands on. So the last three letters of the fern's name are the last three letters of podium, and they were fixed long before anybody was naming ferns.

The last syllable, where botanical Latin keeps letters that speech drops.