My Field Guide to Wildflowers says that cinnamon colored pappus beneath the floral envelope is a good indicator for this species. Here we can see the pappus after I pulled back a bit of envelope.
From Meehans' Monthly:
"The plant seems to have been popular as a medicine with the Indian races. Many Indian remedies have proved no more efficacious than the remedies of the white man, but it is worth noting that much credit has been given to this plant for its virtues, by many modem observers. Pursh was so impressed with its value, that he gave a drawing of it in his " Flora of North America," and says : " this plant is known by the inhabitants under the name of ' Lion's foot,' and is in high esteem as a specific in curing the bite of the rattle- 'snake. During my travels through the mountains of Virginia, I had the opportunity of being a witness of the efficacy of this remedy. A man living in Cove Mountains, near the Sweet Springs, was bit in the foot by a mocassin snake, a species considered the most dangerous. An inflammation and swelling of his whole leg took place immediately, but by taking the milky juice of this plant boiled in milk, inwardly, and applying to the wound the steeped leaves, which were very frequently changed, he was cured in a few days."
Meehan, Thomas. Meehans' Monthly, Volume 3, 1893. 162. Web. Google Book Search. 9 Aug 2009.