(Chapter 14) Footnote 7:

Another muted quarrel with Galenic anatomy: De usu partium 4.110.12-14 (May p. 603) states only that there is a cartilaginous epiphysis at the end of the sacrum, from which it may be inferred that Galen’s observation was based on young animals. See May p. 574 n. 46. The 1555 edition omits even this tacit challenge to Galen, saying only that the coccyx is composed of four ossicles. In modern anatomy, the tissue between sacrum and coccyx is recognized as a fibrocartilaginous disc, not an epiphysis; but in the young there are epiphyses between the four bones of the coccyx and the five bones of the sacrum.