The discovery of the pneumatic condition of the vertebræ in Ornithosaurs and certain Dinosaurs showed that they diverge from Reptiles in structural characters which are typically Ornithic. The augmented number of vertebræ in the sacrum in both those groups also shows a divergence from existing Reptiles, which is markedly Avian. On the other hand, the small number of sacral vertebræ found in the Archæopteryx has proved that a bird may have the sacrum no more complex than in Ornithosaurs and Dinosaurs. The bird's sacrum formerly had a simpler structure, just as the intervertebral articulation was simpler; and we expect to find some of the distinctive osteological attributes of existing birds wanting among earlier representatives of the class. This would also be a fair inference from Prof. Huxley's exposition of the sacrum of the fowl. Sacral vertebræ are defined by their nerves uniting to form the sacral plexus; judged by this test, the fowl has five sacral vertebræ. But ossification has extended beyond them, so as to incorporate in the sacrum the four anterior vertebræ which were originally dorso-lumbar, and the five posterior vertebræ which were originally caudal. When the number of vertebræ is reduced, the structure of the sacrum may be simplified. At the present day the most striking character of a bird's sacrum is the absence of transverse processes extending outward from the bases of the true sacral vertebræ, so that deep depressions are formed in its middle region, comparable to the entire sacrum in some Ornithosaurs. These depressions