politrichum, one letter out
heard as politrichum
spelled polytrichum
One letter apart. The second letter is y, not i: polytrichum.
What the ear did
The Greek upsilon in polys arrives in English as y, and it does the same job in polygon, polymer and polyester. Nothing in the sound tells you that, since the vowel here is a plain short i. You have to know the word is Greek.
P-O-L-Y-T-R-I-C-H-U-M
The full entry for polytrichum covers the name itself.