222 THE VERTEBRATE SKELETON. 



THE SACRAL VERTEBRAE. 



The sacral vertebrae are two in number, they are short 

 and wide, their centra are ankylosed together, and their neural 

 arches are not united to the carapace. 



The first has the anterior face of the centrum concave and 

 the posterior flat, while both faces of the second are flat. 

 Each bears a pair of short ribs which meet the ilia, but are 

 not completely ankylosed either with them or the centra. 



THE CAUDAL VERTEBRAE. 



The caudal vertebrae are eighteen in number. The 

 centrum of the first is flat in front and is ankylosed to 

 the second sacral; behind it is convex. The others are all 

 very similar to one another, and decrease gradually in size 

 when followed back. Each has a moderately long centrum, 

 concave in front and convex behind, both terminations being 

 formed by epiphyses. The neural arch arises only from the 

 anterior half of the vertebra ; it bears a blunt truncated 

 neural spine and prominent pre- and post-zygapophyses. The 

 first seven caudal vertebrae bear short ribs attached to their 

 lateral margins, the similar outgrowths on the succeeding ver- 

 tebrae do not ossify from distinct centres, and are transverse 

 processes rather than ribs. 



B. THE SKULL. 



The skull of the Turtle is divisible into the following three 

 parts : 



(1) the cranium ; 



(2) the lower jaw or mandible ; 



(3) the hyoid. 



(1) THE CRANIUM. 



The cranium is a very compact bony box, containing a 

 cavity in which the brain lies, and which is a direct continua- 

 tion of the neural canal of the vertebrae. 



