(Chapter 5) Footnote 22:

Hippocrates describes four types of skull in the beginning of De capitis vulneribus, but he does not mention Vesalius’ fourth type of “unnatural” skull as illustrated in his fifth figure. Evidently Vesalius did not have the Hippocratic work in hand and misread (or had a variant text of) Galen De usu partium 3.755.4 (May p. 460). Our text of Galen indeed denies that this variant is possible, going on to say this is why it is not mentioned in Hippocrates.