172 THE VERTEBRATE SKELETON. 



In the tail region of other forms each vertebra consists 

 of an anterior centrum bearing the neural arch, and a pos- 

 terior intercentrum 1 bearing chevron bones. Vertebrae of this 

 type are called embolomerous. Haemal arches similar to the 

 neural arches are often found as in Urodela. The transverse 

 processes are sometimes well developed and are divided into 

 tubercular and capitular portions. 



In Gymnophiona the vertebrae are biconcave and are very 

 numerous, they sometimes number about two hundred and 

 thirty. Only quite the last few are ribless and so can be 

 regarded as post-thoracic vertebrae. The first vertebra has 

 nothing of the nature of an odontoid process. 



In Anura the number of vertebrae is very greatly reduced, 

 only nine and the urostyle being present. Of these, eight are 

 presacral and one sacral. The urostyle is postsacral and 

 corresponds to three or more modified vertebrae. The first 

 vertebra is without transverse processes, the remaining pre- 

 sacral vertebrae have the transverse processes fairly large, 

 while the sacral vertebra has them very large, forming in 

 some genera widely expanded plates. The urostyle is a long 

 cylindrical rod which articulates with the sacrum generally by 

 two facets. Ankylosed to its anterior end are the remains 

 of two neural arches. 



In Anura remains of the notochord are found in the centre 

 of each vertebra, i.e. vertebrally, while in the Urodela they 

 only occur intervertebrally. 



The vertebrae in Anura are, as a rule, procoelous. The 

 eighth vertebra is however generally amphicoelous, while 

 the ninth commonly has one convexity in front, and two 

 behind. 



In some forms such as Bombinator, Pipa, Liscoglossus and 

 Alytes they are opisthocoelous ; in others like Pelobates they 

 are variable. 



1 See p. 14. 



