(Chapter 16) Footnote 28:

Three tubercles replace the transverse process of the twelfth thoracic vertebra: the lateral tubercle is similar to a true transverse process, the superior tubercle is similar to the mammillary process of a lumbar vertebra, and the inferior tubercle is similar in function to the accessory process of a lumbar vertebra.

The 1555 edition explains the diminishing size “no doubt because the eleventh and twelfth thoracic ribs are articulated only to the vertebral bodies.” It then omits the two sentences which follow in the first edition.