INTRODUCTIOX. 23 
wing-coverts, 28. There are corresponding lower wing- 
coverts. 
In the Posterior Extremities are distinguished the thigh, 
very short, and generally concealed ; the leg or tibia, 29, 
commonly but erroneously named the thigh ; the instep or 
tarsus, 30, erroneously named the leg, generally bare, and 
having upon its surfac^ plates or scales. Lastly, we have 
the toes, generally four, one directed backwards, 31, and 
named the hind or first toe ; the rest directed forwards, 32, 
and named the anterior, or, counting from within, the second, 
third, and fourth toes. The toes are covered with plates and 
scales, and terminated by claws, or horny sheaths, which 
vary in size and form. 
A few more explanations, referring especially to the bill, 
toes, and plumage, will be useful. 
The Bill in birds is an instrument for the prehension of 
food, and is scarcely ever used for mastication, properly so 
called, that function being performed by the stomach. It 
varies greatly in size and form, according to the nature of 
the food, and accordingly affords some of the most obvious 
characters for distinguishing the different genera and fami- 
lies, which, in fact, may be easily known by the inspection 
of that organ alone. Thus, if we take the bill of a bird of 
the Hapacious order, a Falcon, Fig. 2, for example, we 
Fig. 2. 
shall find it to be as described at p. 48, short, very strong, 
of nearly equal breadth and height at the base, moderately 
compressed, that is, flattened sidewise, toward the end ; the 
