Sauropsida: Class Aves 543 



not with the pterygoids, the two latter bones being movably joined. 

 Skulls having the more reptilian (therefore presumably older) type of 

 palate are called paleognathous; those of the other type are neognathous. 

 On this basis, modern birds are separated into two divisions: Pa- 

 laeognathae, including all of the "running birds" and the flying 

 tinamous; and Neognathae, including all the others. 



Of the many classifications of birds, no two are quite alike. In other 

 vertebrate Classes the major divisions, such as subclasses, usually 

 break directly into a small number (not including extinct groups) of 

 orders which are distinguished by such conspicuous anatomic differ- 

 ences that there can be no doubt about the validity of the subdivisions. 

 Any attempt to split up the major divisions of Aves at once compels 

 recognition of some 20 or more living groups, most of which can be 

 defined only on the basis of peculiarities which seem trivial as com- 

 pared to those which separate, for example, Urodela and Anura or 

 Ophidia and Chelonia. These subdivisions of Aves are somewhat un- 

 fairly given the dignity of Orders, but it is obvious that, in the general 

 scale of classification, they stand more nearly at the level of the families 

 in other classes. In the following outline of avian classification, the 

 so-called "Orders" will not be defined beyond mention of some of the 

 better-known representatives of each and a few of their general 

 characteristics. 



Class Aves 



subclass archaeornithes 



Mesozoic birds: Long tail with many freely movable vertebrae, 

 no pygostyle; teeth of reptilian type (conical) in both jaws. Archae- 

 opteryx. 



SUBCLASS NEORN1THES 



Later Mesozoic birds and all Cenozoic birds: Short tail usually 

 ending in a pygostyle; teeth in some extinct Cretaceous birds, later 

 birds all toothless. 



I. ODONTOGNATHAE : 



Extinct Cretaceous toothed birds. Ichthyornis, Hesperornis 

 (p. 541). 



II. PALAEOGNATHAE : 



[ratitae (p. 542) : Orders 1-4] 



Order 1. C asuariiformes : Cassowary (Casuarius) and emu 

 (Dromiceius) ; ostrich-like, but smaller than African ostrich; wings 

 extremely vestigial; no carina on sternum; three toes present (second, 

 third, fourth) ; aftershaft on feathers; found only in Australia and some 

 neighboring islands (Fig. 419). 



