The ending of polytricham
heard as polytricham
spelled polytrichum
It ends -um. Polytrichum.
What the ear did
The ending is the Latin neuter -um, unstressed and reduced to almost nothing in speech, so the ear cannot tell you which vowel is in there. Every Latin genus name of this shape ends the same way: Leucobryum, Hypnum, Thuidium.
P-O-L-Y-T-R-I-C-H-U-M
The full entry for polytrichum covers the name itself.