In Plate 1 of professor Owen's invaluable memoir on the Archæopteryx (Phil. Trans. 1863) the fifth vertebra behind the Acetabula is seized upon as the first of the caudal series. In Pl. 3, fig. 5, of the same memoir, we have the delineation of a young Ostrich’s pelvis; and in that figure the first post-femoral joint is marked as the commencement of the true tail; eight such joints being, even in the young bird, embraced by the posterior processes of the iliac bones.