Gleanings: Lovecraft on a wrong Roman legend

Lovecraft, in O Fortunate Floridian (p. 57), casatigates the old myth of the games thusly:

"The Sign of the Cross" {Paramount 1932 Cecil B DeMille} was quite a spectacle, even though it gives a sadly one-sided picture of Roman Life - leaving out the sober, normal citizenry altogether. In the arena scenes I noted the persitent error which has haunted folklore for many generations - namely, that downturned thumbs were the signal for the killing of a conquered opponent. How this common mistake ever gained currency I'm hanged if I know, but it seems very deep seated. Actually, upturned thumbs formed the death signal, while waved handkerchiefs were the signal for mercy.