pseudavernia and the vowel at the join
heard as pseudavernia
spelled pseudevernia
There is an e there, not an a: pseudevernia.
What the ear did
The vowel is the first letter of Evernia, the genus name being borrowed. That is also why there is no o after pseud-, unlike pseudonym or pseudopod: the o drops out in front of a vowel, and what looks like a missing letter is a deliberate elision.
P-S-E-U-D-E-V-E-R-N-I-A
The full entry for pseudevernia covers the name itself.