A NEW PHYTOSAUR FROM THE TRIAS OF ARIZONA 157 



arid regions permits a wider range about the limiting water supply 

 to compensate for the diminished food supply and the distance 

 between water-holes. 



In Machaeroprosopus the modifications not only met an emer- 

 gency but actually turned it to advantage; the phytosaur waited 

 for the rigorous conditions to bring his food within reach. If our 

 interpretation of the conditions in the western interior of North 

 America during Triassic times is correct the scarcity of food necessi- 

 tated a wide range for many forms. This meant speedy forms 

 ordinarily difficult of capture, but it also meant the periodic 

 crowding of the limited water-holes; it compelled the food to 

 walk to the captor. 



It is not easy to see why a group so remarkably fitted to their 

 environment as was the phytosaur group should not prosper 

 better as a race. They were very short hved, confined to the latter 

 part of the Trias, apparently. One can scarcely appeal to over- 

 specialization unless this means "fitting in" rather than marked 

 structural change. The phytosaurs were scarcely less generahzed 

 than the living crocodilians. Perhaps perfection alone is enough 

 to condemn a race. A consideration of this possibility will form 

 the basis for a future paper. 



