TORTOISES AND TURTLES. 23 



of horn, covering equally symmetrically arranged 

 bony elements. There is no skin, save that 

 represented by the horny shields, and no inter- 

 mediate layer of flesh. The bony elements repre- 

 sent in part the bones of the shoulder girdle of 

 other animals, and in part the peculiar " abdominal 

 ribs " which we have already mentioned as a 

 notable feature in the Tuatera " lizard." There 

 is apparently nothing equivalent to the breast- 

 bone of other reptiles to be found in the Tortoises. 



The development of the shell has been accom- 

 panied by many other and profound changes in 

 the form of the skeleton. Thus, the backbone, 

 between the base of the neck and the base of the 

 tail, has become virtually suppressed, its originally 

 separate elements being greatly reduced, and 

 immovably fused one with another. The reduc- 

 tion of the spine to its present almost vestigial 

 condition has been a long process. The decline 

 was inevitable directly the bony shell had acquired 

 rigidity enough to prevent the movement of the 

 backbone. As soon as this stage was reached a 

 new order of things became established, and the 

 material of the superseded spine became available 

 for absorption and reincorporation into the newly 

 developing structures. This is the fate of all 

 useless organs in Nature ; they are broken up, 

 like old buildings, to contribute to the growth 

 of new ones. 



Yet another remarkable feature is the position 

 of the limb-girdles. In all other animals the 

 shoulder-blades lie on the outside of the foremost 

 ribs ; but in the Chelonians these bones are placed 

 on the inside of the ribs, i.e. within the shell. 



