178 THE INFANCY OF ANIMALS 



The history of the breastplate is no less strange. Like 

 the shell of the back, this is covered externally by symmetri- 

 cally arranged plates of horn, answering to the skin of 

 other animals, and these plates cover a series of symmetri- 

 cally arranged bones, which answer in part to the bones 

 of the shoulder-girdle, and in part to what are known as 

 " abdominal ribs " — bony rods developed in the fleshy 

 walls of the abdomen, as, for instance, in the tuatera 

 " lizard," and the extinct ichthyosaurs or sea-dragons. 



But this is not all ; for the position of the limb-girdles, 

 as the bony supporting frameworks for the limbs are called, 

 furnishes another remarkable feature. In all other verte- 

 brates, above the fishes, the shoulder-blades He on the 

 outside of the foremost ribs, but in the tortoise tribe these 

 lie under the ribs, and therefore inside instead of outside 

 the body — that is to say within the shell. Similarly, the 

 haunch-bones which support the hind-limbs in other 

 animals lie behind the last rib, and near the surface of the 

 body. In the tortoise tribe they lie, like the shoulder- 

 blades, within the shell. But these curious conditions 

 again obtain only in the adult. In very young tortoises, 

 where the development of the shell has only just begun, 

 the shoulder-blades lie a little in front of the first rib, 

 whilst the hip-bones are to be found just behind the last 

 rib. But the broadening of the ribs caused by the ex- 

 cessive development of the external bony shields gradually 

 creeps over the shoulder-blades in front and the hip-bones 

 behind, so that at last they come to lie, as we have seen, 

 within the shell. 



That this shell was developed to afford a protective 

 armour against predatory animals seems certain, since, 

 when tortoises have no further need of such armature, the 

 shell tends to degenerate, and to enlarge the openings for 



