(Chapter 18) Footnote 23:

In os coccygis, the coccyx, the average number of vertebrae in the dog is 20 (with a range of 6 to 23); in the chimpanzee the number is 3, in the baboon the average number is 19, and in the human the usual number is 4. See Miller 1964, p. 58 and Swindler and Wood 1972, p. 12.