(Chapter 18) Footnote 17:

Galen mentions the coccyx in De anat. adm. 310.5, 11, 15; 311.4, 10; 398.9; 399.8; 533.16; 534.3; 587.11, 14. For the sacrum, see 537.4-5, 585.5-8, and 586.4-6. In the first chapter of book 6, Galen distinguishes between apes which are most like man and have a small coccyx and others with a long coccyx, which “may have a tail.” (533-534, tr. Singer 1956 p. 147). The former would be tailless forms such as the Barbary ape, Macaca sylvanus, Galen's chief referent in projecting human anatomy.