Page:The Osteology of the Reptiles.pdf/130



ribs of reptiles, like those of the amphibians, primitively articulate with all vertebrae, at least as far back as the middle of the tail. The first to become fixed or closely united with the vertebrae, after the sacral, were the caudal, next the lumbar, and last of all the cervical. The dorsal ribs are free, except in the Chelonia, some Pterosauria, and some armoured dinosaurs.



. 86. Vertebrae and ribs: A, Clidastes (Mosasauria), posterior cervical vertebra, from behind; B, Cymbospondylus (Ichthyosauria), anterior dorsal vertebra, from the side, after Merriam; C, Ichthyosaurus (Ichthyosauria), middle dorsal vertebra, from the side (after Broili); D, Dimetrodon (Theromorpha), anterior dorsal rib; E, Diadectes (Cotylosauria), anterior dorsal rib.

The ribs of the Temnospondyli (Fig. 86) articulate with intercentrum and arch, usually without differentiation of the articular surfaces. And this was the original mode among reptiles. With the diminution in size of the intercentrum, the head, or capitulum, joins the adjacent ends of two centra across the intervertebral cartilage, the articular surface, however, continuous to the tubercle, which articulates with the end of the diapophysis. This continuous articulation from the intercentral space to the arch was the almost