THE VERTEBRAE 93 



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, gradu- 

 ally became the prevailing one, all reptiles since early Eocene times, 

 except the geckos among lizards, the turtles, and Sphenodon, pos- 

 sessing 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 doubt- 

 less 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 ptero- 

 dactyls acquired the ball-and-socket articulations after the attain- 

 ment of flight. 



The presacral vertebrae of the sauropod, as also the cervical verte- 

 brae 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 sala- 

 manders. 



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 ( = Amy da) 

 spinifera, sl living river-turtle, has opisthocoelous cervical vertebrae, 



