20 6 
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 
a 
c d 
Fig. 163. — Feathers, a , a quill-feather ; b, quill-feathers fastened to 
wing ; c, a shape-feather ; d, a down-feather. 
recognized points of external structure; this structure 
being dependent largely on the habits of the birds. 
These groups of birds are often spoken of as orders; 
but these orders do not correspond to orders among 
insects. 
