428 REPTILES xv. 18- 



changes that can be recognized in the bony parts available for study; 

 no doubt there were many others in the soft parts also paralleling the 

 evolution of birds, for instance the animals may have been warm- 

 blooded. However, there is no evidence that they possessed feathers; 

 the wing was a membrane (patagium). 



*Rhamphorhynchns of the Jurassic is still recognizably of archo- 

 saurian structure, especially in the skull, which has two fossae and 

 large forward-sloping teeth (Fig. 248). The fore-limb was elongated, 

 but the carpus still short, with an extra 'pteroid' bone in front, pre- 

 sumably to support the wing. The first three digits were short and 

 hooked, the fourth long, supporting the wing, and the fifth absent. 

 The hind-limb was slender, with five hooked digits. There was a long 

 tail, ending in an expanded 'fin'. Both girdles had well-developed 

 ventral regions and there was a large 'sternum', keeled in front. The 

 scapula articulated directly with the vertebral column. 



*Ptera?wdon, of the Cretaceous, showed further modifications. The 

 trunk became shortened to ten or fewer segments and the fore-limb 

 further lengthened, the carpus being long and the fourth digit much 

 longer than the other three. The hind-limb remained small and the 

 tail became very short. The very large and elongated head gradually 

 lost its teeth, presumably acquiring a horny beak. In the latest forms 

 the skull was drawn out backwards into an extraordinary process. 

 Some earlier related forms were only a few inches long, but *Pterano- 

 don itself, of the late Cretaceous, had a wing-span of 25 ft. 



Zoologists have not yet succeeded in reconstructing the life of these 

 animals, and it is hard to see how they could have walked on land. 

 The membrane, which stretched between both legs and the body, and 

 perhaps also included the head, must have been easily torn. The 

 feathers of birds can be ruffled without breaking and the loss of a few 

 does no great harm: the bat's wing can be torn, but at least it is 

 supported by many digits, whereas that of the pterodactyl was a huge 

 continuous membrane supported by a single finger. Again it is difficult 

 to see how the animals can have perched ; if they hung, as the claws 

 suggest, was it with the front or with the hind legs? And how can 

 they have staged a take-off, which in birds is greatly helped by the 

 jump of the hind legs? It is possible that they always came to rest 

 hanging from cliffs, which they could leave by soaring. Even the flight 

 itself presents many difficulties. Although there is a sternum and a 

 strong humerus, neither suggests the presence of muscles sufficiently 

 strong to carry a creature as large as *Pteranodon. We cannot solve any 

 of these mysteries, but one clue is that the biggest pterodactyls were 



