Flower is of the Hymenocallis genus (different species), but it is not a true lily. There are 21 species and this genus was reclassifed in the APG II revison. My best guess is that this flower is of the species rotata - (wheel shaped, almost flat). I believe the rotata link is showing the "floridana" species. I explain this in photo 18. This must be an extremely rare species with flowers being 3/scape, grows in the rivers and spring runs of North Central Florida.
