rytidiadelphis, two letters short
heard as rytidiadelphis
spelled rhytidiadelphus
Rhytidiadelphus, with the h after the r and a u in the final syllable.
What the ear did
The ph has been kept, which is the hard part, and the two easy-looking places have gone wrong instead. Silent h at the front and an unstressed Latin ending at the back are the two spots where this name loses most people.
R-H-Y-T-I-D-I-A-D-E-L-P-H-U-S
The full entry for rhytidiadelphus covers the name itself.