a group must have been intermediate between Odontornithes* I mean such Odontornithes as Ichthyornis: Hesperornis is another instance of degeneration induced by disuse of the wings. and Carinatæ is quite sufficient to account for all the reptilian affinities of the Ratitæ: the assumption that they had acquired the distinctive characteristics of shoulder-girdle and sternum, pelvis, vertebral column, and fore and hind limbs, and had lost all trace of teeth, will account for the fact that in these points the Ratitæ make no approach whatever to reptiles. The same hypothesis also explains the fact that while the Ratitæ agree in certain common characters, some because they are of ancestral importance, others because they have been acquired by the same law of degeneration, they differ in the most remarkable way in other points. For instance, while Dinornis retains such ancestral structures as the hallux, large external xiphoid processes, and free ischium and pubis, it has undergone the greatest amount of degeneration of the shoulder-girdle and forelimb. Struthio, in the same way, shows the extreme of modification in the foot, Struthio and Rhea in the pelvis, and so on. Probably the great size of the Ratitæ is also connected with their cursorial life, and is, like so many points in the skeleton, merely an exaggeration of what is found in Notornis, Cnemiornis and Didus. The Proto-Carinatæ being, by the hypothesis, good fliers, were presumably not of gigantic size; moreover they probably possessed feathers with connected barbs, so that the special characters of the Ratite plumage should be looked upon as a secondary or degenerate, not as an ancestral, character. Finally, if we look upon the typical Odontornithes and Archæopteryx as approximately linear types in the ancestry of birds, we must assume that the latter arose from ornithoscelidan reptiles of comparatively small size, the gigantic Dinosauria being a special development of the same type. The following diagram expresses the results to which a consideration of the above facts seems to lead:—