156 COMPAKISONS OF STKUCTUBE IX AKDIALS. 



side of these flattened expanded processes, and 

 united lo tlaem and to each other by sutm-e, is 

 generally, but not always, a range of longer 

 bones— these are evidently the ribs, as may be 

 clearly seen by looking at the spine, where the 

 way in Avhich they proceed fi-om it is manifest. 

 Here, then, Ave have a large shield, formed not 

 by new elements, not by bones or parts of 

 bones unknown in other animals, but by modi- 

 fications merely of structural detail. But the 

 carapace is not yet complete ; it is margined all 

 round Avith another set of flattened bones, 

 imited to the rib-plates, and generally regarded 

 as the analogues of the sternal ribs of the 

 crocodile.'-' Perhaps they are analogous to the 

 sternal portion of the ribs in birds, Avhich is 

 bony, and not, as in the ordinary mammalia, 

 cartilaginous. f 



The carapace is united at its sides to the 

 plastron, Avhich unquestionably represents the 

 breast-bone. 



Thus modified, the dorsal portion of the 

 spine of the tortoise, it need scarcely be said, is 



* Ribs arising from a prolongation of the long narrow brea.st- 

 bone for supporting the abdomen. 



t In tlio ornitliorbyn.cUus the sternal portion of the ribs i$ 

 osseous as in birds, , 



