rhytidiadelphas, corrected
heard as rhytidiadelphas
spelled rhytidiadelphus
The last vowel is u: rhytidiadelphus.
What the ear did
By the sixth syllable of a name most listeners have stopped tracking vowels at all. The ending is the ordinary Latin -us, the same one on Hypnum's neighbours in every flora.
R-H-Y-T-I-D-I-A-D-E-L-P-H-U-S
The full entry for rhytidiadelphus covers the name itself.