444 



CHORD AT A. 



body is enveloped in this hard protective case, which \i 

 formed of a dorsal carapace and a ventral plastron, joinec 

 together laterally. The carapace usually has a median row 

 of neural plates resting over the neural region, a lateral row 

 of costal plates modified from the ribs, and a distal row o: 

 marginal plates. The plastron seems to be made up of £ 



Fig. 314. — Skeleton of a Tortoise. 



The plastron has been cut away on the right side of the body and thrown ovei 

 to the animal's left. Note the carapace and plastron, flexible cervical and caudal 

 but ankylosed dorsal, vertebrae. 



pair of clavicles and an episternum anteriorly and a num 

 ber of abdominal ribs posteriorly. The " case " of the 

 Chelonia is evidently to a large extent composed of a number 

 of pre-existing structures. Inside the case the vertebrae, ae 

 might be supposed, are vestigial, with the exception of the 

 cervicals and caudals. All four limbs are present and 



