two changes at once
polipodeum finds nothing in a nursery list
The spelling is polypodium.
two changes at once
Two vowels have moved, one near the start and one near the end, and a catalogue search will usually fail on that rather than suggest a correction. The two letters at risk are always the same. The y belongs to poly and the i belongs to podium, and if you can hold on to those two words the rest of the name assembles itself.
The class
Two separate decisions about the letters, both made at once.
Same name
Other spellings recorded for polypodium
The name