OUR BEST FEATURES 



perhaps even with their mouths open, and then 



suddenly engulfing it in a living trap. Such a 



line of specialization leads often to wide flat skulls 



and very shallow, widely-bowed jaws set with rather 



small teeth on the margins and a few larger 



piercing teeth on the roof of the mouth, as in the 



great labyrinthodonts or stegocephalians of the 



Permian and Triassic periods. Others, in which 



the jaws became very long and narrow, actively 



swam in pursuit of fishes. But those amphibians 



{e.g. Fig. 48 II) which were destined to give rise 



to the line of ascent to man, avoiding both these 



extremes, had jaws of only moderate length and a 



skull of moderate width and considerable depth, 



especially toward the rear end. At first they 



retained the teeth on the roof of the mouth (Fig. 



53, II-IV) but in the series of reptiles (Fig. 53, V) 



that finally culminated in the cynodonts (Fig. 



53, VI) and probably in the mammals (Fig. 53, 



VII), the teeth on the roof of the mouth, that is, 



on the primary upper jaw, were eliminated and the 



marginal teeth on the secondary jaws acquired the 



typical dog-toothed or caninif orm type of predatory 



animals that pursue their prey on land. 



From this condition there are intermediate stages 



115 



