From Works of Wonder:
"Since I was endeavoring to be the illustrator most faithful to Burroughs' descriptions, Lester del Rey and I had a lengthy discussion about whether the white lizard I wanted to show had six or eight legs. The Barsoomian counterparts to four-legged creatures on Earth always had eight legs, but Burroughs specifically described the white lizard as having only six. We could not decide if this was a mistake, or if Burroughs intended it to have six legs because it had spent its entire life in isolation in the cave. But the author wrote six, so I painted six."