INTRODUCTION. 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 surface 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 Sill 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 Rapacious 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 



