THE VERTEBRAE 105 



primitively large size, reaching the ventral side, doubtless because 

 of the loss, fusion, or great decrease in the size of the axial inter- 

 centrum. In the marine crocodiles (Fig. 80 h) 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 be- 

 tween 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-hypa- 

 pophyses, serving for the attachment of neck muscles. 



In the Plesiosauria (Fig. 80 a) the odontoid is to a greater or less 

 extent visible from the side, but is much reduced. In both the plesi- 

 osaurs and pterodactyls the atlas and axis are fused, indistinguish- 

 ably so in the adult; both are slender-necked animals with small or 

 vestigial cervical ribs. In the short-necked Ichthyosauria the atlas 

 and axis show a progressive fusion from the earlier forms (Fig. 80 c), 

 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 c). 



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 g) 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 G, h), anomodonts, and some lizards it has disappeared or is 

 represented by the merest vestige. It is small in the dinosaurs and 

 chelonians. 



