OSTEOLOGY OF CHELONIAN REPTILES. 123 



can be retracted within the thoracic-abdominal box; the 

 four limbs are figured as extended in the act of walking, 

 to show their structure. The only movable vertebrae are 

 those of the neck and tail, and the former enjoy a great 

 degree of flexibility. The vertebrae answering to the dor- 

 sal, lumbar, and sacral series are firmly fixed together ; 

 but the dorsal ones, 1 to 8, are chiefly concerned in the 

 formation of the osseous dwelling-chamber. The com- 

 position of this will be first described as it exists in the 

 turtle (cheIone\ the species called "loggerhead" being here 

 selected for its illustration. 



In the marine species of the chelonian order, of which 



isrJa 



SKELETON OF THE EUROPEAN TORTOISE. 



this may be regarded as the type, the ossification of the 

 carapace and plastron is less extensive, and the whole 

 skeleton is lighter than in the box-tortoise (Fig. 20), 

 or any of those species that live on dry land. The 

 head is proportionally larger — a character common to 



