also Catholic Encyclopedia says:
There was a tradition current among the Jews that the skull of Adam, after having been confided by Noah to his son Shem, and by the latter to Melchisedech, was finally deposited at the place called, for that reason, Golgotha. The Talmudists and the Fathers of the Church were aware of this tradition, and it survives in the skulls and bones placed at the foot of the crucifix. The Evangelists are not opposed to it, inasmuch as they speak of one and not of many skulls. (Luke, Mark, John, loc. cit.)