38 ANNALS NEW YORK ACADEMY OF SCIENCES 



In correlation with this active arboreal life the brain became highly 

 developed; the olfactory lobes grew smaller, the cerebra, optic lobes and 

 cerebellum attained large size and high development. The brain-case 

 was correspondingly expanded, while the skull as a whole acquired a very 

 light construction. The earliest birds retained the sharp conical teeth 

 implanted in distinct sockets which were characteristic of their '^theco- 

 dont" ancestors (such as Euparheria) . The teeth and jaws were adapted 

 for quickly snapping at living prey, perhaps insects. These entirely rep- 

 tilian teeth, which in the case of Marsh's "toothed birds" bore an extra- 

 ordinary resemblance to the teeth of mosasaurs, were retained long after 

 the main adaptations for flight were established. 'But perhaps during 

 the Cretaceous period the ancestors of modern birds lost their teeth as 

 the horny beak at the front of the jaws grew backward. 



The skull of birds is of a modified reptilian type and has no doubt 

 been derived simply by the loss of the upper temporal bar, by the inturn- 

 ing of the pterygoid bones and by the enlargement of the internal nares. 

 In short, the whole architecture of the bird skeleton, as indeed the whole 

 internal anatomy, are unqiiestionably a modification of a primitive rep- 

 tilian type. The consensus of opinion is that this ancestral type was 

 nearly related to the primitive Archosauria (Diapsida), or two-arched 

 reptiles, and was very widely removed from the mammals, mammal- 

 reptiles, turtles, plesiosaurs and iehthyosaurs. 



The hypothesis that the ratite birds have come off from some group of 

 reptiles other than that which gave rise to the carinate birds is, in the 

 writer's judgment, entirely untenable : first, because an examination of 

 the skulls of various ratites and carinates shows agreement in funda- 

 mental plan, with minor, though well-marked, differences in details ; sec- 

 ondly, because the entire organization of the ratites indicates merely a 

 cursorial readaptation of carinate types; thirdly, because the tinamous 

 are in many characters allied both to the ratites and to the carinates, and 

 thus strengthen the conservative view that the class Aves is broadly 

 monophyletic in origin. 



