(Chapter 16) Footnote 12:

An additional thoracic vertebra, a thirteenth, results from the formation of ribs on the first lumbar vertebra, with a consequent reduction of the number of lumbar vertebrae. This number may also be reduced with fusion of the fifth lumbar vertebra to the sacrum.

At this point, the 1555 edition adds “[the thoracic vertebrae], which also least resemble each other in all of their parts.” The next four sentences of the 1543 edition are then omitted, down to the section titled “Diversity in the bodies of thoracic vertebrae.”