I have the Fournier edition of Marseilles, which I always compare with the Camoin – Jodorowsky edition. But I will skip the description of the cards, and try to share some of my thoughts about the un-named Arcanum.
There is notice that, in ancient Greece, the god Hermes (Mercury) was sometimes pictured as a skeleton, in order to reinforce his role as guide of the souls in the world of the death – the Hermes Psychopompos, and also Hermes the Wayfinder, as in the Odyssey. Other comparisons between this personification of Death (or of Transition…) and Hermes can be made: one of his epithets was "the quick one", and this name is analysed by some experts as being related to death gods, meaning the one that is "fast as death". Hermes is the mediator between life and death.
About the card’s number, note that number 4, resulting from the addition of 1 and 3, is strongly related to Hermes, and is referred to in very ancient hymns: his “birthday” was in a day 4, and in Argos the fourth month of the year had the name Hermaios. Quotating an author, “the quaternity was for the ancients one of the most constant constituents of the Hermes image; they further acknowledge this in the four-cornered form of the Herms” (“Herms” were carved stones, made for devotional proposes, with the head or heads of the god Hermes, and sometimes also with Apollo; they have a phallic form, but a square base).
He was also very deeply related to the night (being also a “ruler of dreams”) and the obscure aspects of life. Because of this, as is of common knowledge, he become a tutelary figure for what we could call the “hermetic” mysteries. It is also very important to enhance the fact that he learned his divinatory skills with Apollo, and, as far as I can see, this makes him the perfect guide to tarot practitioners…
Obviously, his relation to Tarot is well explored in many senses, but not, to my knowledge, in relation to this Arcanum. Maybe we can find him a bit all over, in tarot cards, namely in the first one (the Magician – he was also the god of magic and illusion) and in the last one (The World - he could be pictured with four heads and four aspects, as a representation of the totality of the universe).
Again about the skeleton: the limits of human life are the mother’s womb, and the grave. Both can be seen here, I think – death is generally accepted as being female, and the earth which “she” revolves is a symbol of the largest of all “mother’s wombs”, were life is continually irrupting from apparently “dead” material. And in the skeleton’s pelvis, the “space” where internal organs should be appears to be emphasised, perhaps to point out the place of a uterus… or am I seeing too much? Any initiation, of course, is a kind of death, for we must die to this world in order to born into another.
The quotation I made belongs to the book “Hermes Guide of Souls”, by Karl Kerényi.