THE SKELETON IN REPTILES. VERTEBRAL COLUMN. 275 



ENDOSKELETON. 



VERTEBRAL COLUMN. 



The vertebral column is commonly divisible into the usual 

 five regions, but in the Ophidia, Ichthyosauria, and Amphis- 

 baenidae among Lacertilia, only into caudal and precaudal 

 regions. In the Chelonia there are no lumbar vertebrae. 



The form of the vertebral centra is very variable. A large 

 proportion of extinct reptiles, several entire orders, and the 

 earlier and more primitive forms in some of the other groups 

 have amphicoelous vertebrae. Vertebrae of this type occur 

 in the Theromorpha, Ichthyosauria, most Sauropterygia and 

 Rhynchocephalia,' and many Dinosauria, also in some of the 

 early Crocodilia such as Belodon, Teleosaurus and Goniopholis, 

 and the Geckonidae among Lacertilia. 



The majority of living reptiles have procoelous vertebrae. 

 Thus they occur in the Lacertilia (excluding the Geckos), the 

 Ophidia, and the Crocodilia, also among extinct forms in the 

 Pterosauria and many Dinosauria. On the other hand some 

 Dinosauria such as Iguanodon have opisthocoelous cervical 

 vertebrae, while others have opisthocoelous thoracic vertebrae. 

 The vertebrae of the Ceratopsidae and some Sauropterygia, 

 the thoracic vertebrae of Iguanodon, and the sacral vertebrae 

 of Crocodilia have flat centra. The first caudal vertebra of 

 modern Crocodilia is biconvex, and in the Chelonia all types 

 of vertebral centra are found. The cervical vertebrae of 

 Sphenodon are noticeable for the occurrence of a small pro- 

 atlas, which may represent the neural arch of a vertebra in 

 front of the atlas. 



In most reptiles the vertebrae are fully ossified, but in 

 some of the more primitive forms the notochord persists in 

 the centre of the vertebra (i.e. intervertebrally), this is the 

 case for instance in many of the Theromorpha and Rhyncho- 

 cephalia, and also in the Geckos. In other reptiles it persists 

 longest intravertebrally. 



The centrum of each of the caudal vertebrae of most 



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