112 THE STORY OF REPTILE LIFE. 



crocodiles alone among modern reptiles, have 

 preserved a complete series of free ribs in this 

 region. The part which these ribs play in loco- 

 motion is briefly as follows. The free ends of 

 each pair of ribs are attached to the ends of 

 one of the broad shields which we have remarked 

 running down the under surface of the body. 

 When the creature walks the ribs are moved 

 forward, one pair at a time, and in doing so, they 

 move the horny shield to which these free ends 

 are attached, so that its hinder edge projects 

 downwards from the body and catches hold of 

 whatsoever inequalities in the ground there may 

 be. These inequalities serve as levers by which 

 the body is thrust forwards. As a further aid 

 to progress the body is thrown into a series of 

 undulations in a horizontal plane, but never 

 vertical, as is sometimes erroneously represented 

 in pictures. 



Mention has been made of limbs. These occur 

 only as the merest vestiges, representing the 

 hind-limbs, in certain archaic burrowing snakes 

 and in the giants among the snakes, the Pythons 

 and Boas. Externally, they are to be found in 

 the shape of a pair of claw-like spurs half- 

 concealed between the scales, at the wider end 

 of the body. Dissection reveals further vestiges 

 in the shape of the remains of thigh bones and 

 hip-girdles. All trace of the fore-limbs, their 

 girdles, and of the sternum, has absolutely 

 vanished. The remnants of these once functional 

 limbs are very precious links to the student of 

 evolution, for they prove beyond doubt the fact 

 that the modern snake has descended from limb- 



