AVES 



411 



pletely surrounded by protective membranes. The cavity 

 between the true amnion and the body wall (Fig. 208, am. c.) 

 is the amniotic cavity and may be filled with a fluid. 



441. Classification of Aves. 



Subclass I. Archcsornithes. — These are extinct birds related 

 to the extinct reptiles — the dinosaurs — in having a vertebrated 

 tail, and jaws bearing teeth. Each vertebra of the tail possessed 



Fig. 210. 



Fig. 210. Apteryx australis. From Romanes. 



Questions on the figure. — What peculiarities does this bird present? What 

 does Apteryx mean? What is the distribution of this species? What are its 

 nearest relatives among the birds? 



a pair of feathers, the tail thus having a row of rectrices on 

 .either side. 



Archseopteryx, of which three specimens have been found in 

 the lithographic quarries of Bavaria, represents the group and 

 was about the size of a crow (Fig. 209). 



Subclass II. Neornithes {modern birds). — This group is 

 characterized by the reduction and fusion of the tail vertebras 

 in such a way that the tail feathers (rectrices) are arranged 

 in a semicircle (or sometimes wanting). Teeth are wanting 

 except in some extinct forms, which stand intermediate between 

 the Archseornithes and the recent birds. 



Division I. RatitcB {flat). — These are running birds with a 



