Once thought to be a single widespread North American species, Adiantum pedatum (maidenhair fern) was recently segregated into three – the eastern Adiantum pedatum, the western Adiantum aleuticum and (known only from Vermont (and Quebec, see comments)) Adiantum viridimontanum.
Although the similarities and differences between Adiantum pedatum and Adiantum aleuticum have been recognized in the past (the western taxon sometimes being known as Adiantum pedatum subsp. aleuticum),
reproductive isolation (they don’t interbreed) and the number of morphological differences drove the decision to ultimately split the traditional species up.
Photographed in UBC’s BC Rainforest Garden, site of a good collection of Pacific Northwest ferns.