74) and Triassic Ichthyosauria and continuous to the present time in the living gecko lizards. More usually, since middle Permian times the cavities are shallow, bowl- or saucer-like, or almost flat (platycoelous) or even quite flat (amphiplatyan).
Until after the middle Jurassic times the vertebrae of all known reptiles were amphicoelous. A ball-and-socket joint appears at that time, so far as we yet know, with the concavity in front, the ball or convexity behind. This kind of vertebra, called procoelous, gradually became the prevailing one, all reptiles since early Eocene times, except the geckos among lizards, the turtles, and Sphenodon, possessing them. Procoelous vertebrae appeared among the Crocodilia in early Cretaceous times (Hylaeochampsa), amphicoelian types, however, persisting until early Eocene (Dyrosaurus). The vertebrae of all known snakes (Fig. 73 e, f), dating from Lower Cretaceous, are procoelous, as are also the presacral vertebrae of the Pterosauria, dating possibly from early Jurassic times. The caudal vertebrae of some turtles are procoelous. Procoelous vertebrae, however, are not restricted to reptiles, some modern frogs having them. They doubtless arose in terrrestrial crawling reptiles with a flexuous column, and it was doubtless from such ancestors that the aigialosaurs and mosasaurs, aquatic reptiles, inherited them. Possibly the pterodactyls acquired the ball-and-socket articulations after the attainment of flight.
The presacral vertebrae of the sauropod, as also the cervical vertebrae of many theropod and orthopod dinosaurs, have the convexity of the centrum at the front end, just the reverse of procoelous. Such vertebrae have been called opisthocoelous, and are doubtfully known in other reptiles, save the cervicals and caudals of certain turtles. They do occur, however, in the cervical region of certain Triassic Stegocephalia, and in some modern fishes and many modern salamanders.
Most remarkable are the cervical vertebrae of the Chelonia. The earliest that we know had amphicoelous vertebrae throughout the column, but most others have an extraordinary combination of all types, amphicoelous, procoelous, opisthocoelous, plano-concave, plano-convex, and even biconvex, otherwise known in only the first caudal vertebra of the procoelian crocodiles. Platypeltis (=Amyda) spinifera, a living river-turtle, has opisthocoelous cervical vertebrae,
