140 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



nosaurs, 1 plesiosaurs, ichthyosaurs, precretaceous crocodiles, 

 geckos, rhynchocephalia, and the fossil birds Archceopteryx and 

 Ichthyornis. 



Opisthocoelous : Lepidosteus,.rs\os\. salamanders, Pipa, Dis- 

 coglossus (anura), most dinosaurs, some vertebrae in penguins 

 and auks, and the neck vertebrae of most ungulates. 



Precocious: the majority of anura, reptiles, and birds. 



In the majority of mammals the vertebrae are flat upon each 

 end of the centrum, amphiplatyan. 



In forms with amphicoelous vertebrae there was no true 

 articulation of the separate elements of the vertebral column ; 

 but with the assumption of pro- or Opisthocoelous conditions the 

 vertebral centra articulate with one another, and frequently 



FIG. 148. Anterior and posterior faces of a vertebra of Python, from Huxley. 

 ns, neural spine ; //z, postzygapophysis 5/2, prezygapophysis ; tp t transverse process ; 

 za, zygantrum ; 25, zygosphene. 



accessory portions are developed to lock the vertebrae more 

 firmly together. Most common of these are what are known 

 as articular processes (zygapophyses). Of these there are two 

 pairs, arising from the anterior and posterior surfaces of the 

 neurapophyses. The anterior or prezygapophyses have their 

 flattened surface turned dorsally so that they can articulate with 

 the ventral surfaces of the posterior process (postzygapophyses) 

 of the vertebra in front. In the snakes and some lizards (igua- 

 nidae) these are re-enforced by articular surfaces developed from 

 the neural spine. On the anterior surface of the base of the 

 spine a wedge-shaped process (zygosphene) projects forward, its 



1 In Camarasaurus the first caudal is convex on either end. 



