DIET-ADAPTATIONS IN VARAN US NILOTICUS. 77 



the latter. But on the other hand the proportions between 

 the greatest interpterygoidal width and the basal length of 

 the skull is in the young V. niloticus as nearly the same as 

 in the adnlt V. salvator as it possibly could be, for in both 

 instances the said relation can be expressed in the following 

 way: the interpterygoidal width is 17 % of the basicranial 

 length (but in the adult V. niloticus only 7 %). 



In consequence of the direction of the basypterygoid- 

 processes of the sphenoid the pterygoidea are in the young 

 V. niloticus much less curved than in the adult one and re- 

 semble in this respect as well, those of V. salvator. The 

 parietal of the young V. niloticus does not project beyond 

 and above the anterior suture of the supraoccipital as in 

 the adult. Here is thus another likeness with V. salvator 

 and so is the continuation of the plain upper surface of 

 the parietal all the way to the occipital suture. In most 

 respects accordingly the skull of the young V. niloticus re- 

 presents the same stage of development as that of the adult 

 V. salvator. From this more primitive stage every individual 

 then gradually advances to the highly specialised type exhibit- 

 ed by the adult V. niloticus with its many characteristics 

 adapted to a molluscivorous diet. 



The explanation of how such an adaptation has been 

 effected may of course as in all similar cases be put forward 

 in at least two different manners. The one would be as 

 follows. A Varamts-lizeirå of the common type acquires the 

 habit of feecling on land-molluscs. The swallowing of the 

 whole animal, shell and all, involves some difficulties and 

 the lizard tries to crush the shell. Such a habit exercises 

 the museles and causes an individual strengthening of the 

 same but causes at the same time a corresponding enlarge- 

 ment of certain bones and in correlation to this an increase 

 of the massiveness of the whole skull. At the same time 

 during the trials to crush the shells the pointed teeth get 

 more or less damaged, but new teeth are continually devel- 

 oping and most probably influenced by the pressure during 

 their development. They respond to this by becoming grad- 

 ually stouter and getting broadly rounded crowns instead of 

 the acute tips. If now this infLuence on the individual could 

 become hereditary and transmitted to the offspring the whole 

 process would be explained. 



