an unstressed vowel
Why is it not spelled dryopteres?
Because pteris ends in is.
an unstressed vowel
In an unstressed final syllable, English gives -is, -es, -us and -as the same faint vowel. Only the source word can tell you which one is written, and the source word is pteris.
The class
An unstressed vowel, flattened until it gives the ear no information.
Same name
Other spellings recorded for dryopteris
The name