ADAPTATIONS 101 



been independently acquired in the four groups. In the ich- 

 thyosaurs and pelagic crocodiles its function may have been to 

 aid in resisting the pressure of the water at great depths ; but 

 what may be its use in birds and pterodactyles is not easy to 

 determine. In the case of birds it was at one time supposed 

 that the function of this arrangement was for the purpose of 

 altering the degree of convexity of the cornea during rapid 

 ascent or descent in the air, but objections have been urged 

 against this view ; and, so far as it goes, the occurrence of the 

 same structure in pterodactyles tends to support this objection, 

 since it is scarcely likely that these reptiles were capable of 

 soaring to great heights in the air. When on the ground, or 

 during repose, pterodactyles certainly folded their wings, al- 

 though the precise manner in which they were then carried is 

 uncertain. From the fact that in the fine lithographic limestone 

 of Bavaria, where complete skeletons and occasionally the im- 

 pressions of the wing-membranes are found, no traces of scales, 

 hair, or feathers have been detected, it is practically certain 

 that pterodactyles had naked skins. If we seek a reason why 

 the specialised pterodactyles discarded teeth in favour of a 

 smooth horny beak, it may be observed that all members of 

 the group probably fed upon fish. Now at first sight it might 

 appear that toothed jaws were better adapted for holding 

 slippery prey than is a toothless horny beak ; but it has to be 

 borne in mind that although toothed jaws would ensure the re- 

 tention of every fish captured, yet they would prove a hindrance 

 to its being quickly and easily swallowed. Possibly, indeed, it 

 may have been necessary for a toothed bird or pterodactyle to 

 resort to the shore before being able to devour its prey ; and 

 if this were were the case, we have an explanation why birds 

 and pterodactyles became toothless. That pterodactyles 

 habitually frequented the sea-shore, if not the open sea, is de- 

 monstrated by the occurrence of their remains in marine strata 

 side by side with those of ichthyosaurs, plesiosaurs, and mail-clad 

 fishes. 



Many reptiles of the normal type, such as crocodiles and 

 ordinary snakes, are excellent swimmers, and some (apart from 

 sea-snakes) will venture considerable distances out to sea. In 

 all of these, however, no special modification, either external 



