Why does pseudevernia start with a p nobody says?
heard as seudevernia
spelled pseudevernia
Because the Greek word pseudes begins with a p, and Latin kept the letter after English had stopped pronouncing it.
What the ear did
Greek was happy to say ps at the start of a word and English is not, so the p went quiet and the spelling stayed put. The same silent letter opens psalm, psyche, psychology, pseudonym and psoriasis, and its cousins do the same job in ptarmigan and pneumonia. Every one of those is a Greek word that English pronounces with the first letter switched off. For a lichen name this matters more than it does for an English word, because the p decides where the name sits in an index and what a search matches. Pseudevernia furfuracea is collected commercially, sold as oak moss or tree moss to the perfume trade, and a supplier who writes it seudevernia will not be found by anyone looking for it properly. If a name sounds like it starts with soo-, check for the p first.
P-S-E-U-D-E-V-E-R-N-I-A
The full entry for pseudevernia covers the name itself.