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114 Sauropterygia articulate with the diapophysis (d) exclusively by a single head, the cervical ribs (Fig. 89 r) exclusively with the centrum, usually also by a single head.

The dorsal ribs of the Archosauria, that is, the Pseudosuchia, Parasuchia (Fig. 88), Crocodilia (Fig. 90), Dinosauria (Fig. 80, and Pterosauria, are double-headed, the anterior ones at least, but both articulations are with the arch or diapophysis. And this mode of articulation would seem to exclude their immediate ancestral relationship to the birds,

in which the head of the ribs articulates with the centrum throughout.

Atlantal ribs, present in all early reptiles, have been lost in modern ones, except the Crocodilia, where they are attached exclusively to the intercentrum, in the ancient Metriorhynchus to both arch and intercentrum. In the Dinosauria, some of them at least, the first intercentrum bears a small rib (Fig. 81).

Axial ribs are more often present, but are lost in not a few reptiles, articularly the Pterosauria (Fig. 80 ) and Chelonia (Fig. 80 ). In early crocodiles the axial rib articulated with diapophysis and parapophysis; in later crocodiles the diapophysial articulation is lost, though a vestige often remains, and the single headed rib has migrated forward on the odontoid.

The dorsal ribs of the Eunotosauria and all Chelonia have expanded to meet or fuse with each other, forming more or less of a carapace (Fig. 91). Peculiarly expanded and overlapping ribs in the posterior dorsal series occur in some of the Theriodontia. In Cynognathus the thirteenth to the seventeenth ribs shorten rapidly and project widely with a remarkable expansion near the proximal end, which overlaps the succeeding rib in a concavity on its anterior border. In the lumbar series (Fig. 92) they lose the free portion of the shaft, ending in wide,