The dorsal vertebræ in Cambridge specimens would appear to make a nearer approximation in number to birds than to Mammals or Lizards or Crocodiles, though Chelonians have few vertebræ in the back. Among Reptiles the form of the vertebra makes some approach to that of the Monitor, Chameleon and Scink. In most Mammals the dorsal vertebræ have the centrum convex, though in the lumbar region its visceral surface often becomes flattened. But though very unlike there is a nearer resemblance to the lower dorsal vertebræ of a Struthious bird. Sacrum. Pl. 10. Case. Comp. Tablet. Specimen. =J= _c_ 4 1-7 Seven specimens are mounted to exemplify the ordinary structures of the Ornithosaurian sacrum. Nos. 1 and 2 have the centrum convex, exactly as in the dorsal vertebræ of the convex type. Nos. 3-7 have the centrum flattened, following in general features the plan of the dorsal vertebræ with flat centra. No. 1 is a vertebra from a sacrum, where perfect anchylosis had not been induced; it has the neural arch well preserved, and shows the sharp suture which united it to the preceding vertebra. No. 2 shows two entire vertebræ and part of a third, which have lost the neural arches but have the centra perfectly anchylosed together. The middle vertebra measures 5/8 of an inch in length, and at the suture from side to side measures more. The surface is smooth, regularly convex from side to side, and gently concave from back to front. The last vertebra shows the articular vertebral surface; it is convex in both directions, and oblique, so that a large part looks upward. The anterior of the three vertebræ is pinched in at the lower part of the sides of the centrum. No. 1 shows that the neural surface of the centrum is deeply excavated, making the neural canal an elongated upright oval. Above the centrum, which forms only the middle third of the articular surface, the neural arch expands on each side into a wedge-shaped transverse process, the lower surfaces are flattened, and continuous with the centrum, while the upper surfaces are flat and horizontal as in birds and Dinosaurs, and form the platform from which arises the massive neural spine. In front the transverse wedge is flattened and compressed, so as to look forward and outward, and in the middle shows a large ovate pneumatic foramen. Behind, the wedge is compressed so as to look backward and downward. The neural spine is massive and forms rather more than half the height of the vertebra. It is flattened with a ridge rising near its base in front and ascending in a concave curve obliquely backward and upward. The anterior parts approximate a little in front, while the small parts posterior to the ridge approximate a little behind. The sides of the neural spine approximate superiorly, and appear to round into each other.