Owing to the now classical investigations of Gegenbaur and Frenkel, and the more recent researches of other observers, the several homologies of the vertebral column are distinctly understood. The specific or individual differences in the correlation of one region of the column to another can be adequately explained on the assumption of a suppression or excessive development of the potential costal element of the vertebral segment. This costal element may be metamorphosed in different ways to suit the needs of the animal economy, and the variations in individual cases affect the segments at the ends of a series where the vertebrae of one region possess characters resembling those of a neighbouring region.