We have in New England three taxa in the genus Polypodium, two species and their hybrid. They are:
Polypodium appalachianum (diploid)
Polypodium virginianum (tetraploid)
Polypodium X incognitum (triploid)
P. virginianum is actually alloploid, resulting from a chromosome-doubling mutation of the hybrid between P. appalachianum and P. sibericum, which is native to the Paciific Northwest.
New England Polypodium collections were initially named P. vulgare, a European species which looks similar. Around 1900 the name was changed to P. virginianum and herbaria slowly began to use it. The distinctions among the three taxa were eventually discerned around 1940. It was later determined that the tetraploid taxon was the specimen named P. virginanum by Linnaeus in 1753, so in 1991 the term P. appalachianum was taken for the diploid. The hybrid was finally described in 2002.
There had been no distinction made in herbarium records, at least at the Gray and NEBC collections, until Les Mehrhoff collected P. appalachianum in Connecticut in ___ and I brought in Massachusetts specimens of P. appalachianum and P. X incognitum in 2002 as part of our survey of Wachusett Mountain.
P. virginianum has been reported for almost every county in New England, the exceptions being in the southeast: Block Island and Bristol in Rhode Island and Nantucket in Massachusetts. It is possible that some of these reports actually refer to one of the other taxa.
I have undertaken to collect P. appalachianum and P. X incognitum from every county in New England. It may take a while, as there are 67 counties, requiring 134 collections. I have made some progress, with three collections before 2010, 22 during 2010, and 21 in 2011, and one in 2013.
Here is a map showing which New England counties have reports of the two newer Polypodium taxa, with my collections marked with upper case letters, and other county collections marked in lower case. And here are two maps from neatlas.org that show the distributions. (The Maine and Connecticut P. appalachianum collections were not mine.)