THE PHYLA HEMICHORDATA AND CHORDATA 



leave a record as a fossil than is a heavier animal. The usual habitats and 

 habits of birds also make it less probable that these forms would fall to earth 

 in places where fine sediments were being rapidly deposited. More fossils of 

 birds may be discovered at any time, but it is unlikely that their record will 

 ever approach that of animals more commonly buried in the mud of some 

 shallow body of water and thus fossilized. 



The class Aves is divided into the subclass Archaeornithes, containing the 

 extinct forms with teeth and other conspicuous reptilian characteristics 

 {Archaeoplery.x), and the subclass Neornithes, including in four superorders 

 the remaining known birds, extinct and existing. The Odontognathae are the 



Fig. 1 8.27. Dinosaurs and flightless 

 birds. A, Strutkiomimus, a bipedal 

 dinosaur of the Cretaceous. B, the 

 North African ostrich, Slruthio camelus. 

 Many of the features of Struthiomimus 

 were strikingly similar to the char- 

 acteristics of modern flightless ratite 

 birds; however, these similarities are 

 examples of convergence in evolution, 

 for these forms do not appear to be at 

 all closelv related. {A, redrawn after 

 G. Heilmann, The. Oriom of Birds, 

 copyright 1927 bv D. Appleton and 

 Co., printed by permission of 

 Appleton-Century-Crofts, Inc ; S, 

 photograph courtesy New York Zoo- 

 logical Society.) 



577 



