THE SKELETON OF THE TURTLE. THE VERTEBRAE. 221 



THE THORACIC VERTEBRAE. 



These are ten in number and are all firmly united with 

 the ribs and elements forming the carapace. 



The first thoracic vertebra differs from the others, the 

 centrum is short and has a concave anterior surface articu- 

 lating with the centrum of the last cervical vertebra, and a 

 pair of prezygapophyses borne on long outgrowths. The 

 neural spine arises only from the anterior half of the centrum, 

 and is not fused to the carapace. Arising laterally from the 

 anterior part of the centrum are a small pair of ribs each of 

 which is connected with a process arising from the rib of the 

 succeeding vertebra. 



The next seven thoracic vertebrae are all very similar, 

 each has a long cylindrical centrum, expanded at the ends, 

 and firmly united to the preceding and succeeding vertebrae. 

 The neural arches are flattened and expanded dorsally, and 

 are united to one another and to the overlying neural 

 plates ; each arises only from the anterior half of its respective 

 centrum, and overlaps the centrum of the vertebra in front of 

 it. Between the base of the neural arch and its successor is a 

 small foramen for the exit of the spinal nerve. There are no 

 transverse processes or zygapophyses. 



To each thoracic vertebra from the second to ninth in- 

 clusive, there corresponds a pair of ribs (fig. 36, 6) of a rather 

 special character. Each is suturally united with the anterior 

 half of the edge of its own vertebra, and overlaps on to the 

 posterior half of the edge of the next preceding vertebra. 

 The ribs are much flattened, and each is fused with the cor- 

 responding costal plate, beyond which it projects to fit into 

 a pit in one of the marginal plates. 



The tenth thoracic vertebra is smaller than the others, and 

 its neural arch does not overlap the preceding vertebra, it 

 bears a pair of small ribs which are without costal plates, but 

 meet those of the ninth vertebra. 



There are no lumbar vertebrae. 



