The articular surfaces between the bodies of the vertebræ, in the neck, are transversely oval. The middle part of this articular joint is made by the body of the vertebra; its outer parts are in the neural arch. In front this surface is a hollow channel, often more depressed than in any other animals. The corresponding surface behind is convex, with a process on each side at its lower outer angles (Fig. 25). It is a modification of the cup-and-ball form of vertebral articulation, which at the present day is eminently reptilian. Serpents and Crocodiles have the articulations similarly vertical, but in both the form of the articulation is a circle. In Lizards the articular cup is usually rather wider than deep, when the cup and ball are developed in the vertebræ; it differs from the vertical condition in pterodactyles in being oblique and much narrower from side to side. Only among Crocodiles and Hatteria is there a double articulation for the cervical rib, though in neither order have rib or vertebra in the neck the bird-like proportions which are usual in these animals. Pterodactyles show no resemblance to birds in this vertebral articulation. A Bird has the corresponding surface concave from side to side in front, but it is also convex from above downward, producing what is known as the saddle-shaped form which is peculiarly avian, being found in existing birds except in part of the back in Penguins. It is faintly approximated to in one or two neck vertebræ in man. Professor Williston remarks that in the toothless Pterodactyles of Kansas the hinder ball of the vertebral articulation is continued downward and outward as a concave articulation upon the processes at its outer corners. There are no mammals with a cup-and-ball articulation between the vertebræ, so that for what it is worth the character now described in Ornithosaurs is reptilian, when judged by comparison with existing animals. [Illustration: FIG. 25. CERVICAL VERTEBRA OF ORNITHOCHEIRUS From the Cambridge Greensand]