Page:The Osteology of the Reptiles.pdf/123

Rh primitively large size, reaching the ventral side, doubtless because of the loss, fusion, or great decrease in the size of the axial intercentrum. In the marine crocodiles (Fig. 80 ) the pleurocentrum is more reduced. Among the Chelonia the atlas may fuse into an independent vertebra, articulating with the axis. At other times the odontoid is more or less united with the axis, with no motion between it and the ring of the atlas. The axial intercentrum may be paired or single, fused with the odontoid or apparently absent. When paired they are more or less elongated, forming pseudo-hypapophyses, serving for the attachment of neck muscles.

In the Plesiosauria (Fig. 80 ) the odontoid is to a greater or less extent visible from the side, but is much reduced. In both the plesiosaurs and pterodactyls the atlas and axis are fused, indistinguishably so in the adult; both are slender-necked animals with small or vestigial cervical ribs. In the short-necked Ichthyosauria the atlas show a progressive fusion from the earlier forms (Fig. 80 ), in which a complete disk represents the atlas, to those in which the bodies of atlas and axis are imperfectly or indistinguishably fused (Fig. 80 ).

Axis (Figs. 78-81). The axis differs from the following vertebrae in its broader and stouter spine, its usually more elongated centrum, and in its relations with the atlas. Its prezygapophyses are small and turned outward at the base of the spine. In the Cotylosauria and Theromorpha the front end of its centrum is deeply concave, the persistent notochord continuous through the notochordal odontoid. In procoelian, opisthocoelian, and platycoelian vertebrae the front end is flattened for sutural or ligamentous union with the odontoid. Its centrum is usually longer and usually bears a rib, though in the modern cocodiles (Fig. 80 ) and the dinosaurs (Fig. 81) its rib has migrated forward.

The axial intercentrum is nearly always present, primitively larger than the following intercentra, and is intercalated between the bodies of the atlas and axis in the usual way. Among the crocodiles (Fig. 80, ), anomodonts, and some lizards it has disappeared or is represented by the merest vestige. It is small in the dinosaurs and chelonians.