sudevernia: how many letters are missing?
heard as sudevernia
spelled pseudevernia
Two, both at the front. Pseudevernia begins pse-.
What the ear did
The p and the e are inaudible together, so the syllable arrives as a bare soo and gets written that way. Greek pseudes gives English a long line of words that all start with those two letters and none that pronounce the p.
P-S-E-U-D-E-V-E-R-N-I-A
The full entry for pseudevernia covers the name itself.