"The number of limbs is the most obvious distinction," remarked John Carter. "In my autobiographical writings, which I arranged for Edgar to publish as if they were fiction, I made it perfectly clear that Barsoomian white apes, like the Barsoomian green men, had their three pairs of limbs articulated in three different ways. The first pair of limbs worked just like human arms; the last pair worked just like human legs, and the middle pair were like the forelimbs of Earthly dogs or horses, able to support weight in a four-footed walking gait, yet also equipped with prehensile digits, thus able to serve as extra hands. It shouldn't have been hard for comics and movies to depict what I described; but EVERY SINGLE ADAPTATION produced on Earth has cheated! They always depict the upright non-human creatures of Barsoom as having FOUR IDENTICAL ARM-TYPE ARMS, making no attempt to show how those intermediate limbs would work."
As Captain Carter was frowning over this, the present-day science-fiction author David Weber came by, greeted the others, and said to Carter: "For what it's worth, I imagined an intelligent alien race called the Treecats of Planet Sphinx for my 'Honor Harrington' series. Treecats have six limbs formed just like those of the Barsoomian apes; and in my stories, I take pains to describe the DIFFERENT operations of the three sets of limbs."
John Carter found that he did feel better after hearing this. As a sign of appreciation, he drew his absurdly long Barsoomian longsword and cut Mister Weber's head off -- knowing that this would be no more than a joke, because in this roleplay Mister Weber would be alive again in the next post. And he would be able to boast of being beheaded by John Carter of Mars: a suitable boast, because Weber did in fact like to make references in his books to old-time science fiction.