ADAPTATIONS 99 



In the extinct pterodactyles (Ornithosauria) the whole or- 

 ganisation, both external and internal, is modified for the 

 purpose of true flight in the air, and there are consequently 

 many superficial resemblances to birds. Nevertheless, all these 

 are adaptive, and not indicative of avian affinities ; these 

 reptiles having probably branched off from the common stock 

 of dinosaurs and crocodiles independently of birds. The head 

 is very bird-like, having a long beak, which was probably in all 

 cases sheathed with horn, although in the earlier forms (as in 

 primitive birds) the jaws were furnished with a series of 

 pointed teeth. On the other hand, the wings were membran- 

 ous and more like those of bats, being attached not only to the 

 greatly lengthened fore-limbs, but also to the sides of the body 

 and the hind-legs, as well as to the base or the whole of the tail. 

 In the fore-limb there were only four digits, the one corre- 

 sponding to the thumb being absent. Of the four digits the 

 three inner were short and carried sharp claws, probably 

 employed in clinging to rocks or possibly branches ; the outer- 

 most digit, or the one corresponding to the human little finger, 

 was, on the contrary, enormously elongated, and alone carried 

 the wing. In the earlier long-tailed forms the tip of the tail 

 was furnished with a racket-shaped membranous expansion, 

 which probably served the purpose of a rudder in flight. Ap- 

 parently, however, it was found better to entrust the steering to 

 the wings alone, for in the later and specialised forms, notably 

 those without teeth, the tail is rudimentary. The largest 

 species of which the skeleton has been discovered is Pterano- 

 don longiceps, of the upper Cretaceous rocks of Kansas the 

 expanse of wing being no less than 14 feet, or greater than 

 in the largest albatross, while the head, which was extended 

 much behind the line of the backbone, was about a yard in 

 length. In P. ingens of the same formation the expanse was 

 fully 22 feet. The pterodactyles of this genus are remark- 

 able for the fact that the " white," or sclerotic, of the eye was 

 furnished with a ring of bony plates similar to that found in 

 the eyes of birds and in those of the extinct ichthyosaurs, as 

 well as in certain extinct marine crocodiles (Geosauiidcs) ; 

 such a structure being unknown in the smaller and earlier 

 members of the order. Evidently, this bony sclerotic ring has 



