THE SKELETON OF THE FROG. SPECIAL VERTEBRAE. 153 



All the transverse processes are terminated by very small carti- 

 laginous ribs. 



SPECIAL VERTEBRAE. 



The first vertebra is a ring-like structure with a much 

 depressed centrum. It bears in front two oval concave sur- 

 faces for articulation with the condyles of the skull, while the 

 centrum is terminated behind by a prominent convex surface. 

 There are as a rule no transverse processes, and the post- 

 zygapophyses look downwards and outwards. Occasionally 

 however transverse processes do occur. Projecting forwards 

 from the centrum is a minute process better developed in the 

 Newt. This resembles an odontoid process, and it has hence 

 been supposed that the first vertebra is homologous with the 

 axis of mammalia, and that the atlas of the frog is fused with 

 the skull. 



The ninth vertebra has very stout transverse processes 

 directed backwards and somewhat upwards. They articulate 

 with the pelvic girdle and hence this vertebra is regarded as 

 the sacrum. The neural arch is much depressed, the centrum 

 is convex in front and bears on its posterior surface two short 

 rounded processes for articulation with the urostyle. 



The urostyle is a long rod-like bone forming the posterior 

 unsegmented continuation of the vertebral column. It is 

 probably equivalent to three vertebrae, the tenth, eleventh, 

 and twelfth fused together, and to an unsegmented rod of 

 cartilage which lies ventral to the notochord. The anterior end 

 is expanded and bears two concave articular surfaces by means 

 of which it articulates with the sacrum. A prominent ridge 

 runs along the dorsal surface, but gradually diminishes when 

 traced back. The anterior portion contains a canal which is 

 a continuation of the neural canal. At a point not far from 

 the anterior end, this canal communicates with the exterior 

 by a pair of minute holes which correspond with the iiiter- 

 vertebral foramina. 



