168 THE STORY OF REPTILE LIFE. 



this organ, in order that we may study it a little 

 more closely. 



As in the rest of the skeleton, this organ shows 

 signs of increasing complexity when traced from 

 the earliest known specimens onwards to the 

 time when they finally disappeared; though 

 these changes are not of a very marked character. 

 Of its origin we know nothing ; but of the five 

 digits which we may justly assume this limb 

 originally possessed, only four remain. The 

 missing digit is that answering to our little 

 finger. The first, second, and third fingers were 

 short, and armed with claws, which served pro- 

 bably to suspend the animal when at rest, whilst 

 the fourth was enormously elongated. This 

 served, during life, for the attachment of a large 

 fold of skin extending outwards from the body, 

 and backwards, to include the hind-limbs and tail. 

 It differs fundamentally from the wings of the bat 

 and bird. In the former, all the fingers are pre- 

 served, the thumb is free, and the remaining four 

 are enormously elongated and extremely slender, 

 serving to support a thin skin-fold much as the 

 ribs of an umbrella support its covering. In the 

 bird, three fingers remain, the thumb, which is 

 short, is free, whilst the second and third digits 

 are closely bound together, forming a rod for 

 the support of ribbon-shaped organs known as 

 the quill-feathers. Further details of this wing, 

 compared with that of the bat and pterodactyle, 

 by the way, will be found in the " Story of Bird 

 Life." 



As in the bird and bat, there extended along 

 the front of the wing, from the shoulder to the 



