ADAPTATIONS TO SPECIAL ENDS 137 



poise to the beak, although if large-billed birds like the adjutant 

 stork can manage to get on without such a structure it is diffi- 

 cult to imagine why it should be necessary in the case of the 

 pterodactyle. Another feature of this giant pterodactyle is the 

 presence in the socket of the eye of a ring of bones, comparable 

 to those found within the eyes of birds. The existence of this 

 " sclerotic ring " of bones in the eye of the larger pterodactyles 

 is an instance of a parallel adaptive character, for it is unknown 

 in the smaller members of the group, and since pterodactyles 

 have no relationship with birds, it must have been acquired inde- 

 pendently in the two groups. Another instance of the same kind 

 is the loss of the teeth in both these groups ; the more primitive 

 birds, like the oldest pterodactyles, having the jaws armed with a 

 full series of sharply pointed teeth. Pterodactyles of all kinds 

 doubtless fed, like gulls and cormorants, on fish, and at first sight 

 it might seem that toothed jaws were better adapted to hold such 

 slippery prey than is a smooth horny beak. It has, however, to 

 be borne in mind that although such toothed jaws would ensure 

 the retention of every fish captured, yet they would prove a 

 hindrance to its being swallowed quickly and easily. Possibly 

 it would have been necessary for a toothed bird or a toothed 

 pterodactyle to resort to the shore before being able to devour 

 its prey ; and if this be the case we should have an explanation 

 of the reason why both birds and pterodactyles discarded teeth 

 in favour of a horny beak, which enabled them to bolt their 

 food while on the wing. 



The dinosaurs of the iguanodon group have undergone a 

 still more remarkable development in connection with the for- 

 mation of a beak, for in these herbivorous reptiles an additional 

 and separate toothless bone — the predentary — was developed at 

 the tip of the lower jaw, and formed with the equally toothless 

 premaxillai, or anterior paired bones of the upper jaw, the 

 muzzle. Whether in this instance the beak was sheathed in 

 horn may be doubtful. 



A further stage is presented by the horned dinosaurs, such 

 as Triceratops, of the Upper Cretaceous strata of North America, 

 in which a separate bone (the prerostral) was developed at the 

 tip of the upper jaw, so as to correspond with the predentary 

 below ; and it is generally believed that in these reptiles a horny 

 sheath cased the entire beak. 



