28 WATER REPTILES OF THE PAST AND PRESENT 



is not needed. The arrangement of the dentine in primitive reptiles 

 is complicated, that is, plicated or folded in labyrinthine figures, 

 like that of many stegocephalian amphibians, the Labyrinthodontia, 

 especially. This labyrinthine structure of the dentine persisted 

 longest in the ichthyosaurs. 



VERTEBRAE AND RIBS 



The spinal column or backbone of reptiles, as in all air-breathing 

 vertebrates, is made up of a variable number of separate segments 

 called vertebrae, permitting flexibility. Each vertebra is com- 

 posed of a body, or centrum, and an arch on the dorsal side for 

 the protection of the spinal cord. Various projections from the 

 vertebra, called processes, serve for the attachment of ligaments 



Fig. 12. — Procoelous vertebra of snake: zo, zygantrum; zi, zygosphene; />2, pos- 

 terior zygapophysis. 



or muscles, for articular union with adjacent vertebrae, or for the 

 support of ribs, and these processes have characteristic differences 

 in different reptiles. The pair in front and behind, for articulation 

 with the adjoining vertebrae, may become obsolete or even lost in 

 swimming reptiles, as we shall see; they are called zygapophyses. 

 In not a few reptiles there is an additional pair for zygapophysial 

 articulation in front and behind, called zygosphene and zygantrum, 

 for the greater strengthening of the column; they are especially 

 characteristic of snakes and certain lizards. In certain other 

 reptiles, especially the long-necked dinosaurs, there is an addi- 

 tional pair arranged differently from the zygophene, that have 

 received the names hyposphene and hypantrum. 



On the top of the arch is the spine or spinous process, which 

 may vary enormously in size and length; sometimes it is flattened 



