206 ANIMAL ACTIVITIES. 
Some fossil birds had teeth like reptiles, and at least 
one bird had a long reptilian tail (Fig. 166). Because 
of these resemblances birds and reptiles have sometimes 
been classed together as Sauropsida. 
Classification of Birds. Birds have been most com- 
monly classed in accordance with their most easily 
Fic. 163.—Feathers. «, a quill-feather; 4, quill-feathers fastened to 
wing ; ¢, a shape-feather ; d, a down-feather. 
recognized points of external structure; this structure 
being dependent largely on the hahits of the birds. 
These groups of birds are often spoken of as orders; 
but these orders do not correspond to orders among 
insects. 
